Blood on my hooves - How I got so awesome (Warning: Sarcasm)

Character development. It's really, really tricky. In fact, almost everybody seems to have problems with it. I'm not saying I'm doing it perfectly and everybody else is wrong. No no, there are people out there who are far better at it than me. But still, you have to give your characters some personalities. How do?

In my early works (Which happen to be the first that got onto FIMfiction...), you could notice that there was little depth to many ponies, especially when they were not a main character.
I'm a bit biased about the topic, but I think that it's easier to make your character's decisions comprehensible if you understand them yourself. I must watch out not to ramble. Alright.
Okay, structured. You must give your character strengths and weaknesses, fears and worries, joys and talents. Only when they act like a real person would and an attentive reader could guess their reactions even if they're not completely obvious. That requires you to have made a clear personality for them, as well as being able to think like they would.

Here's where the Bleeding Effect comes in. No, this is not related with anything in my works that happens to share its name. What my personal Bleeding Effect does is that, if I really want to, I can temporarily take somebody else's mindset. Using that, I can ask myself 'What would X do now?'. I've always been good at empathising with people. Some time, one or two months ago, I've managed to perfect that into the Bleeding Effect. It's been around before that, too. If I let myself get too immersed in something I was reading, I would become the character in that story that was the most like me. Fortunately, it didn't last long, as I couldn't control it.

You can probably imagine what a useful tool this is as an author. It allows me to serve as a vessel and think the situations my ponies go through as themselves! As soon as I get Splitting aMid the Night your way, people will probably tell me what a shallow personality Midnight has, but I still think that it is used to its greatest effect in there. I admit to having given Midnight some traits of myself, though she's far more than that. But I couldn't avoid that if I wanted. Bleeding doesn't work 100%, and I don't think I want it to. If it would, I could forget entirely who I am in the process.
So, a bit of my own personality remains whenever I bleed into somepony and that transfers some of myself onto them. But even after disabling the Effect, some traces of them remain within me, if only for a limited amount of time. Add the passive Bleeding Effect and you've got the recipe for a lot of chaos.

Not that it wouldn't be handy, of course. During the creative phase of writing, I create my fantasy world, populate it with ponies, give them what personality I want, set the time and then start bleeding. Although those that aren't the main protagonist like Twilight in An unknown sensation or Midnight in SatN usually don't get bled into, but brought to life with simple empathy.
It's like I'm a standard brony playing with dolls even though I don't own any of that kind. But I swear, if they bring out a purple one with a blue and/or pink mane, I will buy it, recolor its cutie mark and name it Midnight, so help me God. Yes, Midnight has a very specific eye color. No, I won't tell you which one. This is a detail you must imagine yourself.

Where was I going again? Oh yes. I actually got some praise for making Glimmer from On wings of Glimmer an, apparently, normal personality. She's got some depth and isn't a mary sue either (a perfect person, as I'm told). So this is basically the message of this rambly rant. Don't use cardboard cutouts for characters, but when you're giving them life, don't forget to add some downsides to them, too. Also, if you're making an OC, don't make yet another self-insertion. And even if you do, decide on Earth, unicorn or pegasus. I've seen far too many alicorn self-insertion OCs where they just ship their favorite pony with themselves.

Because that's not what OC means! OC means Original Character, in other words, a pony you thought up. It doesn't have to be yourself, it can be anything! For that matter, many ponies you'd think are canon are actually OCs. I think that them being Background Ponies doesn't mean anything at all, as only you give them a personality and hopefully some development.