I don't know about other artists or people who like to tell stories, but I think a lot of people tend to have lots of original characters when they're creating stories and worlds. I am one such person. I have tons upon tons of characters. Of course, not all of them are meant to be stars or main characters in whatever universe they're in, but that doesn't mean I like them less than my more developed characters.
So there is a bit of a problem when you start to need to know these little side characters. How do I get to know them? Normally they would just grow up by themselves through interactions with the "main" characters or with other characters in general. But what about those characters that have pretty much normal backstory? No tragic past, normal grades and schooling. Nothing unusual to speak of. How do you get to know someone like that? How do they develop a personality? It seems to me that you'd have to just explore their life. Find the little moments that would've shaped them into the person they are.
This was (and is) the first major roadblock for my development with Project GC!! It's essentially a game where I took a bunch of side characters and cast them as mains. It's been interesting trying to get into their heads. Naturally the more active characters have been easier to write, but the more minor ones I've been struggling to get to know. I remember my orignal intent or what their essence/purpose should be when they were first created, but when you're trying to write a BxG game you need to delve past the plot related roles of the character and find who they are as a person if you want to show your audience a good character.
I guess the best thing for me would be to take my time and learn about everyone again. I wouldn't rush trying to get to know someone in real life. Trying to get to know anyone takes time. I just need to be more patient. (Ironic statement considering the plot for this game...)