One of my clever non-pony writer-bros shared this list with me some years back, and it's been an invaluable tool not just for defining characters, but also for keeping them in character as they develop. It's an old idea that predates the internet and exists in a hundred variations, but this particular flavor has done right by me.
Every character you write should be able to answer these questions, even if very few of them are relevant in any way that will show up in the story and others are no-brainers. A good story should be a window into a much bigger bigger world that exists out of sight beyond the page, and characters are no exception. I use it on canon (or otherwise "borrowed") characters too, and it's done me no end of good at curing some of my more troublesome writing afflictions.
What's the character's name?
What's their nickname?
What's their gender?
How old are they?
What's their educational background?
What's their vocation or occupation?
What's their social and financial status?
What's their marital status?
What is their family ethnicity?
What is their diction and accent like?
How are their relationships? (ie: Friends, family, ect.)
Where do they live and what is their home like?
What kind of personal possessions do they have, and what is their attitude toward such things?
What kind of hobbies do they have, and what do they do for recreation?
What are their obsessions?
What kind of personal beliefs and values are important to them?
What's their political stance? (ie: What are the hero's views on the evil empire, ect.)
What's their sexual history?
What are their personal ambitions?
What are their religious beliefs?
What sort of superstitions do they hold, and which ones just "get" to them?
What are they afraid of?
What sort of attitude do they carry themselves with?
What are their fundamental flaws?
What are their basic character strengths?
What sort of pets do they have or like?
What is their personal taste in books and music?
What is their preferred taste in food, and what's their favorite dish in particular
What does their hand writing look like? (Weird one I know, but think about it. LOL) -
What's their astrological sign?
What sort of special talents do they pride themselves on, and which talents are they ashamed of? And are they actually as good as they think?
When I brought this up on the chat last night, some folks sort of expressed an interest in seeing it. Ultimately, if anypony at all finds this idea useful, then this blog post has done its job. If not, then I'm howling at the wind with no regrets. Ride on, my crazy ponies.