For me, what happens to characters as a story progresses depends solely on what I discover about them as I go along ----- how they grow, in other words. Sometimes they grow a little. If they grow a lot, they begin to influence the course of the story instead of the other way around. I almost always start with something that's situational. I don't say that's right, only that it's the way I've always worked. If a story ends up that same way, however, I count it something of a failure no matter how interesting it may be to me or to others. I think the best stories always end up being about the people rather than the event, which is to say character-driven. Once you get beyond the short story, though (two to four thousand words, let's say), I'm not much a believer in the so-called character study; I think that in the end, the story should always be the boss. Hey, if you want a character study, buy a biography or get season tickets to your local college's theater-lab productions. You'll get all the character you can stand.
It's also important to remember that no one is "the bad guy" or "the best friend" or "the whore with a heart of gold" in real life; in real life we each of us regard ourselves as the main character, the protagonist, the big cheese; the cameras is on us, baby. If you can bring this attitude into your fiction, you may find it easier to create brilliant characters, but it will be harder for you to create the sort of one-dimensional dopes that populate so much pop fiction. /// - Stephen King, On Writing: 'A Memoir of the Craft'