It’s probably a bit of both for me.  When I come up with an idea for a book, I generally sit down and start brainstorming.  Part of that is coming up with characters, and I make a lot of notes about the characters.  My next step is to organize and formalize the ideas, and I usually do this by outlining the sequence of events and doing a bit of a write up about the characters.  What inevitably happens, though, is that the characters develop on their own throughout the story.  I start writing and then I see where it takes me.  Sometimes the characters end up the same as I originally envisioned but more often than not they evolve a bit as I write the story and develop them more thoroughly and in depth.  Sometimes the story sort of dictates how the characters develop and who and what they become.  It’s always very interesting to see what becomes of each character, how they change, and how they grow.