You start with what you know, but writing is a process of discovery. It's not as if we're simply taking a plate out of our heads and laying it down on paper and printing from it; we're not. Writing is an act of discovery. The moment you start--you start with a character, you start with maybe yourself or maybe somebody else, but you start with a character and you learn about that character by writing about that character and you wind up in places you never thought you were going to go when you started out. I mean that's the whole reason for writing. Discovery. You do not know what you yourself think until you put it into words.
--American Masters series on PBS, Garrison Keillor: The Man on the Radio in the Red Shoes