i was thinking that i could start a thread sharing how you write the Princesses. Please disregard extensive headcannoning. But at the same time try to include things about them like, the way they talk, emotions, motives, physically how they feel, and things like that.
I just had this idea when i was writing today and couldent think of somthing, so to help me and probably many more, share how you describe and write things, so we can look at prespectives and possibly learn from it.

Im not going to, but feel free to also include Twilight and Cadance. Personally, we dont have much for Twilight, and i think Cadance doesent have much of a personality (Don't try to correct me, I wont give a shit). but regadless I dont ever include either one very much.

CELESTIA:
Now, unlike Most people, i say that a full Alicorn's mane is radiation, that excess magic is literally ebbing out of their head. also, Celestia can perform complex spells by encasing somthing in her mane. (IE. banishment, Innoculation, Mind reading, and "Displacing" a person).

When she speaks, she of course acts regal, but also she has the personality of a Kindergarten teacher with her students. that they think she is so far superrior that they worship everything she does, even if she has to descipline one, she has to remember that "they are still babies".

But on the contrary, I believe that Celestia has been ruling for so long that she is actually more comfortable in public and doing things for others than with her own family. Kind of like a soldier who comes home after a tour and cant adjust to a normal life. as a brief example, lets say that she is usually embarassed to be around Luna, but is more than willing to clean up her mess, in a "please forgive my dear aunt sally" way.

Now, when it comes to affection, after living for countless years, Celestia has lost all cares for dignity, even if it can get a little creepy. I dont nessecarily mean Molestia, but meaning that she is not afraid to stand near or touch twilight, or share a bed with Cadance when she was younger. Think of it like in the second episode how when the other ponies were bowing, she instead nuzzled twilight.

Physically, people have done different things with Celestia's size, but i commonly make a point that she "Towers" over everyone else. that she is so strong that, when scenes are focusing on her, she is often worrying that she would hurt others if she doesent keep herself in control, like in my story when she slaps my main character across the face in a fit of anger and is sent across the room, spitting out blood from thier lip and rubbing the swollen side of its face. Otherwise, whe commenting on her fur, I dont think may others do this, but I say that her coat is incredibly soft and warm, almost like a baby blanket.
Motivation wise, I believe that Celestia would fuck up everything in her personal life to serve her country. I'd go on, but i believe you understand what i mean. Other than that, while not afraid to resort to violence like i said above, she is often holding herself back to not cause excessive harm to her opponet. Like in my story durring the wedding, when the Changelings are encasing her in the cacoon, She could easily turn the chapel into the surface of the sun, but is too afraid of the collateral damage. So instead, she makes her body temperature so much that it begings to melt the casing, but is low enough that the ones encasing her can easily repair it, she doesent go any higher in fear that it would kill a changeling.

Weapons Wise, I know a ton of people Prefer Skoll, but i personally see her using a long golden Katana, the way it is meant to be used involves the samaurai to only strike a few times, being effecient with each movement, and ending the fight as quickly as it began. Also, the katana is a beautiful weapon. But just like Celestia, if you make the user pull it out, you better start running.

Like i said before, there is a difference between a Full power and a regular alicorn, and unlike almost everyone else, I say Celestia's natural mane color is a golden blonde, but she uses magic to change it pink.
See this pic for my reason

Furthermore, I believe Celestia is Very protective of Luna, even if she is embarassed. Like the fact that Celestia would hold the moon and sun in harmony for 1000 years (which is incredibly dangerous and taxing, even for an alicorn) Or thier first fight with Discord, who Celestia got her ass whooped because she demanded Luna to stay back. Centuries later, when they fought Sombra, Celestia looked incredibly worse for the wear because Sombra was superpowered and alsohow Celestia was too eager to protect Luna that it was almost hindering their fight.

Counter to most people, even though she is sad about Luna and what she did, She is not nearly as much of an emo as people crack her up to be. while she was remorseful within the first years, she took a more optomistic approach later and kind of put herself on cruise control for a mellenium, always looking forward for her sister, and at one time almost being cold towards everything else.

Other than that, I think that Lastly, Celestia will try almost everything within reason to protect not only her ponies, but everything on her plannet. Like a thing from a long time ago, a starving island nation tried to attack equestria and take land. The Princess swiftly defeated them, annexed their lands to equestria, but instead of enslaving them, she left the order to follow Equestrian laws and princples, pay taxes, and she in turn sent food.

lastly, with government, I say that she has a very brief understanding of economics, but knows a great ammount about poli sci. She allocates fiscal policy to someone who is qualified and takes their advice, but personally she is more keen in knowing how to work with politicians and how to motivate people.
Also, as a bit of personality, she has a small weakness for cake. And also she despises tea, or any warm drink because it reminds her of a sadder time in her life, before Luna's return, Celestia would almost always drink icewater or lemonade. After her return, to fool her sister into thinking she liked tea, she pretends to drink it, but in actuality she makes her lips so hat that the tea evaporates, not actually drinking it. Also, when she is angry and alone, she uses packages of tea as sports shooting skeet to relieve stress.

LUNA:
To put this in as few words as possible, Luna is a bitch with a sense of entitlement.

She is a child, thinking that she she has been granted a position as a god above the mortal ponies, she is more keen to be much more calculated.
She talks, even if she doesent mean it, like she is above you, talking down at everyone besides her sister. in the modern times, she tries to break it, but after years upon years of doing so, its no use. Also, I HATE the Royal We because it is Royally annoying and a Royal Pain in the ass. But that is not saying that when she slips up or gets angry that she cant regress into the Royal we.

Anyway, continueing, I often always protray Luna as the antagonist, because she simply does not know to care for anyone besides herself. She made her guards wear armor that made them look like bats, thinking it would boost morale and make them look fearsome, but behind her back they all bitch about how they look like fools.

In the past, she was the commander of the army, but after banishment, she lost that position because Equestria adopted a modernized military structure.
I usually describe 2 Lunas, the half and the full powered one.
The half powered one is the one we see in the show, with the blue and stary mane. She uses a alchemical shampoo to make he rmane look like her sisters' but it is just fake. her mane is the blue.
When she reaches full power though, she is as tall as Celestia, she can change her colors so that she no longer looks like Nightmare moon, and to be less fearsome she changes her mane to look like the Aura Borealis.
When describing Luna's coat, unlike Celestia, beign warm and soft, Lunas is silky and smooth. its cold, and its slick like ice.

Also, Luna, being a child, (I say this because Celestia had to mature much faster and a lot sooner than Luna, also Celestia is thousands of years older) Luna believes that she can still fix mistakes that she has done, like a kid trying to cover the stain on the carpet, or saying that she didnt know what was wrong.
This is a big thing, I think Luna sleeping all day is bullshit. I say that Alicorns don't NEED sleep, but it does help them. She is awake durring the day, and she she does the majority of the things durring the day. Ill explain more below.

Luna uses a Flacion, also.
So, in short, Luna is not EVIL... she is just not nearly mature enough for what she is doing.
In my story, The whole "Night neglection" thing was a bullshit ruse by Celestia, to hide the real reason why Luna turned into Nightmare moon. while she does appreciate her night and loves it. she also sleeps durring the night. Without it she would seriously not be able to get anything done because there would simply not be enough time. to be honest, the whole "No one appreciates me!" does work, but over her night? kind weak... I changed it around in my story. so i wont spoil it.
Anyway, we cant comprehend how it works, but when an alicorn is injured, they need to heal, and a way to do it is to hide in side of their element. Luna was trapped inside the moon with Nightmare moon, a split personality, and after 1000 years, she was convinced that upon her return form healing, Celestia was going to kill her. the elements seperated Luna and Nightmare Moon, the latter of which is locked away with her power, and Luna must regain her power to become a full alicorn again.

In my mind, Celestia's key characteristics are those: Her duty comes before anything.
And she has understood that the key to being a good ruler is being a balanced person, so she embraces her common needs and whims as something to be tended to. (Little pranks, physical closeness aso.)
She clearly embraces a Socratic kind of education with her students, being more of a guide and helper than an outright teacher.
Most importantly though, she has understood this wisdom: While NOW will never return. Not for her, nor for her people, but she knows that she always has a tomorrow, so she would sacrifice the moment for them.

Luna is a bit different:
She is certain of her status as princess, but not so much of how it is accepted, so she hides behind etiquette and mannerisms.
She is clearly the more serious of the two, maybe even to a fault. I wouldn't wonder if she is a bit jealous of Celestia being older, yet able to remain childish in certain ways.
A part of me thinks that Luna also REALLY likes gloom and gothic. Seen her bat-cape? And no, not that Hot-Topic wannabe goth crap, I mean Gothic. High halls, vertical ornaments and tall shadows cast by moonlight.

I don't have a firm headcannon for the Princesses. In my current Celestia-heavy story I have them as powerful magic users, but not head and shoulders above everyone else in sheer oomph. I might even have Twilight as having more sheer raw power than Celestia, now that they're both alicorns, but Celestia and Luna have tens of millennia of experience and can do things Twilight's never heard of, so regardless of magical muscle they're fantastically skilled. But for a different story I might make them nearly physical gods, if it works better for the story.

What is firmer to me is their personalities, or at least Celestia's. I see Celestia has almost totally dedicated for her little ponies, something she'd compromise only for Luna's sake, and if push came to shove she'd exile Luna to the moon (again). I also see Celestia as wanting to keep in touch with the common pony, but (getting more into headcannon here) she recognizes that the common pony feels better if the one raising the sun every day is an uncommon pony. Would you rather have Abe Lincoln in charge of the sun, or your neighbor?

Luna is harder to pin down, personality-wise. She wants to be more in touch with the common pony, but has mixed feeling about that. She's protective of her little ponies, but, headcannon-wise, I'd be OK if she does that more out of devotion to Celestia. Not that I think Luna is a bad pony at heart by any means.

I write Celestia with a motherly personality. She's very fair, encouraging, playful, kind, etc. but like mothers sometimes she gets tired and needs a break from her job to escape. She prefers to be casual with her subjects and after centuries of ruling Equestria, formalities bore her. At times she may be whimsical and when planning, she plans out far ahead.

Luna I write as the more serious sister. She has something to prove and that is she is nothing like Nightmare Moon was. Unfortunately she can get hot tempered and lose her cool, but she does love her subjects too. I'd almost say I gave her a "fatherly" type of personality where she teaches her subjects to mature and be honest, hard working, and practical. She's more about the "Here and now" with her planning.

Hmm, interesting thread topic
Since the story I'm writing that focuses on Celestia deals with her past, that's where I'm going to stay.
My take on Celestia (I call her Empress!Celestia to differentiate between her and Canon!/Present!Celestia): Things like the wavy mane and her physical size most likely deal with her power levels and age (I say most likely because I haven't thought of a proper justification yet).

Her tone when speaking really depends on the listener: When speaking to Luna she acts more along the line of a mother/ older sister/ teacher, trying to guide Luna and make sure she doesn't get herself hurt/ killed, when speaking to her commanders and soldiers she is authoritative (think stern teacher), and when confronting enemies her dialogue is usually laced with thinly veiled threats.

Affectionwise, She does build upon mentor/student and familial bonds, but she doesn't show any romantic interest in anyone (her first priortiy is governing Equestria, and any distractions might prove detrimental). Nuzzles and hugs are welcome, but anything further than that might draw issues.

Physically, there isn't much to say; she's taller than everypony else, stronger than everypony else, etc. and so-forth (she may be faster, but prefers Teleportation). If she wanted to, she could send someone flying with a well-placed jab, even when she's not wearing battle armor.

Motivationwise, I did state earlier that she does place Equestria first, and she is violently protective of her sister and her subjects. At this point in her history, she is willing to beat down and kill her opponents to ensure that the threat is neutralized, including burning one of her enemies alive (not saying who) for Mind Raping Luna.

Weapons for her focus around her magical/psychic powers, backed up with a Power Sword for close combat (Non-Warhammer players, a power sword is a sword encased in an energy field that cuts through armor like it was tissue paper). I describe it as a longsword or greatsword, because those types of swords often have an association with medieval knights, warlords, and kings, and they practically scream power and authority to the untrained masses (well, depends on your view of power and the region you're from)
That's my take on past Celestia

Hmm. Well, that's actually a good question. Let me see...

When it comes to Celestia, I tend to write her as a rational optimist. She thinks before she acts, she always seems to be one step ahead, and no matter what, she believes that everything will be okay in the end. There are rare exceptions to that (see my story War Games of the Mind for an example), but it's basically that combined with the Elements of Harmony boiled down to their true essence. She's not afraid to speak her mind, she's always there for her subjects and provides them with whatever they need, she is affectionate to those close to her, and, of course, she enjoys a good joke.

With Luna, it's different. There are some aspects that she shares with Celestia, such as her dedication to her subjects, her comforting nature (especially in the dream world), and even a subtle playful side. However, while Celestia is usually confident, I write Luna as more of a worrywart. She's not a complete pessimist, but she can't handle the prospect of things going wrong. Another difference is that while Celestia plans ahead, Luna thinks more on her feet.

Shiny! Can't believe nobody else thought of a thread like this before.

I've always seen Celestia as the fairy-tale queen. The impossible ruler who actually sticks to her principles and follows through on what she promised to do before gaining power. Furthermore, instead of trying to seize or control power by force or fear, she purposefully tries to decentralize Equestria as much as possible. She holds her throne because she earns the love of her ponies, and keeps her nose out of their private business!

This is all based on show canon. We see from Episode 1 onward that she is genuinely affectionate toward her ponies. They bow out of respect, not fear. They're afraid of disappointing her, not incurring her wrath. When she walks among them, she has a token guard at the most. She might well have vast executive power over the cities, but she doesn't try to micromanage. Each city is expected to tend to its own affairs and get along with its neighbors. Cloudsdale certainly seems to handle all the Equestrian weather without input from Canterlot!

This presents a paradox. Celestia, clearly depicted as a crowned ruler, is acting like a representative who honestly expects to be held accountable by the populace. Why?

Because she's a dreamer. She believes that in the soul of each little pony is the potential to do great things. That she, for all her immortality and power, is no more perfect than a newborn foal. Everypony makes mistakes, and everypony has a right to pursue life, liberty, wisdom, and virtue. That's the pursuit of happiness. She has seen the evil that lurks in every soul, and she knows that it lurks inside her as well. She learned that lesson when she lost her little sister. Not even Alicorns are perfect.

Her role is not to rule. She is the Sentinel. Her duty is to safeguard Equestria from all threats, foreign and domestic. It's the duty she was born for, after all, she and Luna did reclaim Equestria from Discord. They battled Sombra, and probably had any number of grand adventures together before the Final Battle. We all know what happened there. Celestia won the duel with Nightmare Moon and banished her with the Elements of Harmony. If that wasn't enough to teach her that power corrupts, and absolute power corrupts absolutely, then nothing would. She's a warrior princess. You don't keep Equestria intact for a thousand years without some battles. She's got gryphons to the north, dragons to the south, and her little ponies are the only bait in town.

This theory had a lot more credence before Chrysalis kinda... well... yeah.

I actually wrote a rather detailed analysis of Celestia's character and her style of ruling into a story I published a while back. Rather than restate it all here, I'll just link it in case anyone wants to check it out. Note that this link goes to the second chapter of the story, the beginning of Celestia's part. If things seem a little unclear, the first chapter will probably clear matters right up!

http://www.fimfiction.net/story/63801/2/love-is-its-own-reward/a-reply

I'm not saying this is the "only correct interpretation" of Celestia. Far from it! This is just my view of her, and how I prefer to write her. We have enough corrupt tyrants in stories. Why not, for once, a girl who sticks to her principles and manages to hold her land together for a thousand years by instilling the benefits of liberty?

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It's possible to salvage that view of Celestia by taking the position that Celestia's power is a blunt object. She can use small (relatively speaking...probably somewhere in the top quartile of unicorns) amounts of power without tapping into the sun. But in the defense of Equestria, she taps into the power of the sun. She can incinerate legions and turn deserts into seas of glass. But she can only scale it down so much. She can't use it against Chrysalis without killing all the innocent ponies nearby.

Basically, I view her as being like a soldier who can use his hands, a knife, a pistol, or a W-80.

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or it could be like i said. Celestia will protect her subjects, resorting to violence. In a part of my fic, a dragon recounts his eyewitnessing of Celestia going into combat for the first time in hundreds of years, how she summoned fires so hot they theirs scales began popping off. But she did not kill a single one of them. we can use that explanation with liek what I said about Chyrsallis, She underestimated Chrysallis' power with the love of Shining armor (even she didn't know how powerful she had become) and she was bested, possibly she even was incapacitated for a while because of the magic backlash?

By no means do i think Celestia is a pussy. She just needs to use her power. She is the only person holding herself back. In my fic that i am writing, Celestia finally accepts that not everything can be solved with negotiations, and with that, she orders an assassination.
But, does that nessecarily mean she has turned in to a tryant? no.

It kind of depends on what story I'm writing. I try and stick reasonably close to canon, but different aspects are emphasised.

Vengeance of Dawn is, in many ways, not a fic that would be welcome here. Celestia there is a schemer, willing to do anything for the sake of her beloved Luna, even cast aside a pony who thinks the world of her because she no longer has a use for her. It is not done with great cruelty, but it is a cruel act nonetheless. Despite her aura of divinity being emphasised (her mere presence reduces the above cast out pony to tears of joy), Celestia is more vulnerable and mortal here than in the show, her attempts to make amends for the mistakes of a thousand years ago cause new mistakes, and despite her best efforts Celestia cannot prevent the disastrous sequence of events that flow from that: all she can do is confess her mistakes and beg forgiveness and understanding.

In In the Shadow and the Light Celestia is a bit more back on form. Though still overly protective of Luna (she goes to war with an interstellar empire rather than send Luna into exile) she is far wiser and more subtle. This is much more the Celestia of the show: always in control of the game as she manipulates events across the world for the good of all creatures. She still makes mistakes, but is much more proactive in correcting them and more successful at it.

This Celestia is also a warrior. Canterlot Wedding is kind of hand-waved away, restoring Celestia to the position of Most Powerful Being (there is a cheeky bit near the end planned where Chrysalis, watching the confrontation between Celestia and an Alicorn Empress, muses that if they fought they could destroy the entire planet. Despite this she rarely gets the chance to fight as her ponies are so protective of her that they refuse to advance until she has retired to safety. This is painful for her, almost as painful as having to send ponies she loves like Twilight into danger on her behalf.

Its interesting, so far the princesses have only been popping up as background characters in my stories. Friendship is eternal only has Celestia as a walk on role and Rarity just mentions Luna in her dream. Luna only pops up at the end of 'Equestria's Mirage' and since then they havent done much. Wich surprises me since Celestia is my fave princess and I love to work with Luna.

But if I were to write for the them (and that fic is coming) I'd say

Twi: The same as I always write her, except just a little more powerful and a little wiser.

Cadence: I like to write as a playful young adult (she is a love being after all) that will turn firce at a moment. I also seem to love playing her as a girl who will be very protective and a healer.

Luna: Utter BADASS! I seem to take the time to make her the polar opposite of Celestia when I can. Though she is loving as her, I also seem to turn her into a princess more on the side of tradition and rules while still employing her fun gamer side. I also like to write her as somewhat good with kids, despite her scary appeance.

Tia: If I write Luna as a stirct and focused teacher on the side of a sensei who will give the students more on the spot training, Celestia to me is more of the teacher that will give the students clues to the answer and let them work it out. While at the same time, will also make the student figure out the reasons why a thing is. I also prefer to make her much more motherly, treating our young twi as the daughter she never had.

Hm... How do I write Celestia and Luna?

... Oh, wow... I can't properly contribute. Outside RPing on a different site, I've never written them, and even then, it's mainly been Luna, and in that RP, I gave her up because I ended up writing her as my mouthpiece in an RP, and that was a no-no.

Celestia is... a mother figure, in that she is nuturing, but firm. That's... all I got. She is the voice of logic and reason between the two sisters.

Luna is passion and naivety, in part due to her exile. I feel she, at this point, would not be an ideal 'teacher', and while her big sister has honed the art of planning ahead, I think of Luna, in turn, as a "think on your hooves" gal.