My characters just sort of pop into my head with their life stories. I honestly don't know how they come to be, but they come to me to tell their stories.
My characters are born off the top of my head. Sometimes their personalities are inspired from other characters I read about in books and then sometimes my characters are born from some of my favorite artists or musicians.
My character are, by definition, people already in existence. But who I make them become stems from anything. Television shows, movies, random thoughts, observations, my personal demons.
My characters come straight from my imagination. I'm inevitably influenced by the things around me, but when it comes down to it, my characters are purely mine, purely their own.
They come from a need. Mainly, I start a story, first, and then from there, I'll think of what I need to fit the roles-- then that is where the characters come in.
Then I'll develope them from there-- I based a lot of characters for a work on songs-- to go a Guilty Gear route in planning things. Interesting approach, it worked, though, the piece of work was a big hit.
I am thinking about doing it, with the newest of my works, but it wouldn't work out with my main character, so I might go with the ordinary way I do things.
I usually get mine off the top of my head, but alot of them are based off of friends, or family members. Some of them I just create when I am falling asleep..
Looking at people. I like basing my characters' appearances off of real people with my own details. I usually get the personalities from books and music. And there's something about watching stand-up comedians that makes me wanna' write.
My character from the story, Jimmy, is really based on the 'Jesus of Suburbia' video from Green Day. First it started with the same name, and then it became much like the person from the plotline.
They usually are loosely based on characters from movies, books, comics ect. I just give them their own personality, really. I kinda of think of them as theur own people, I really just put a name to a face and then they aren't mine anymore, but they belond to themselves. Like Vlad, he was based loosely on Nightcrawler from X-Men Evolution, only a bit older and more angsty. I gave him a name and his appearence, I gave him a setting and his history, and now he pretty much writes himself. Characters are a huge part of the story, for me. So they're very important.
My characters come from anywhere, really. Sometimes, I base them off real people, but sometimes, I get inspired by seeing people on the street. I start thinking about their lives, and what they're like, and my characters grow from that. Mostly, I get my characters before I get my plot.
(Yes, I write fanfiction, but the characters are not the same in each story.)
Most of my characters are need-based. They come about because I have some need I can't fulfill. I manage to get my need fulfilled through them. (Sometimes.)
When it's fanfiction, obviously the characters are people that actually exist. But who I make them come from anything. From how I would see them, television shows, CDs, that kind of thing. I already have some kind of outline seeing as they actually exist, but it needs to be filled in. They sustain their character in each different setting...with maybe a minor change here and there.
And when it's original, they just really come off the top of my head. But mostly it's music and books. When I wrote Cathedral, I was thinking that the character was basically everything and everyone, there was no description of the person nor a name - it only referred to them as "he" - and I thought that was quite a good idea. But music-wise, I could listen to a song and if it's talking about someone else, such as Metallica's "The Unforgiven" then I could develop a character who wishes to get away from society, which is what that song is about.
To be completely honest, I start writing before I really have a character. I set the scene and then just listen for the voice that's stuck in that scene and go from there. From there, I just let my character develop. I was never good at planning things out nor would I be really good at having journals or twitters for my characters. I have a direction I want to go in and let the character lead the way.
tl;dr, the character comes from the story and they wouldn't work at all in any other plot line because the character is the plot line, attached to it in a creepy kind of way.
Their environment, which is usually what comes first when I sit to write something. Then as I write a story, I get to know their personalities, if that makes sense. What they like, what they hate, how they react to what's going on around them.