There are so many sources that add into the pool over the years - people watching, books, TV, films, etc. - that it's hard to say. sometimes they just start out with one single trait or characteristic and stay like that for weeks or months until I can get a handle on what makes them tick. Sometimes that trait will lead naturally into building them up as a more complete character.
Dane (from Dane & Joe) was originally going to be a simple character with no complexity. He's slowly evolving into something bigger. Because game characters have to be bigger from the start, much of the working up goes on off-screen so that I know what makes them tick the moment they make their first appearance.