Well, most of my characters are real people, so I know quite a bit about them, but when a character isn't based on a real person, and many of my protagonists are not, I know nothing about them, and don't want to know anything except what comes out as a logical part of the story.
I really don't care in the least when a character's birthday is, what his favorite food is, or much of anything along these lines. If his birthday comes up as part of the story, then, fine, it is when it is. If he goes into a restaurant, he'll order whatever he wants to eat, and without me knowing in advance what that will be.
I treat such characters just as I would a person I just met. I know nothing about someone at first meeting. I learn about them over time, by observing them, and by whatever they choose to tell me somewhere down the line.
Real people surprise me, shatter my expectations of what I think they will or won't do, do or don't like. I like my main character to do the same.
And, really, since I never do outlines, character charts, or anything else along these lines, how else could it be? When I start a novel I know nothing about it except the title and genre. Then I just sit down and start writing. I don't think about first, plan nothing in advance, so I really can't know anything about such a character, at least until and unless he does it, or tells me.
