I don’t, mostly because when left to my own devices, I’m at the extreme “figure it out as I go along” end of the writing spectrum.
What I do to figure out my characters is … I start writing. As I write, especially in early drafts, I slowly get a feel for who my characters are. In later drafts, I take what I’ve learned in earlier ones and apply it to my characters more consciously–and more consistently.
Sometimes I’ll have a hint of a character’s voice when I start, in my opening lines. But mostly character, like plot, is one of those things I figure out as I go along. This is one of the reasons revision is so important to me–I’m a much stronger rewriter than writer, and need to get something very messy down on paper first. Revision is a crucial part of the whole process for me.
Sometimes I suspect that in my early rough drafts, I’m doing the same work that outliners and those who work out their character backgrounds ahead of time do, only in a very different sort of way.