They KNOW stuff.
Me, I never do. I seriously start out with a keyboard, a blank screen and a blindfold and I fumble and and I stumble and I run face first into the wall a few dozen times before I begin to understand the landscape I'm working with and the people I am creating.
I've tried the other methods--truly I have. I've done the character interviews. I've made the charts, the sticky notes, the vision boards. But until I see my characters in action, until I watch them maneuvering in their unknown world, I don't get to know them.
Like I said, it's a wonder that I ever finish a book with so much ambiguity in my beginning. But I guess that's what keeps me coming back for more.
How about you? Do you know where you're going and who you're taking along before you start writing?