It all sort of builds up. Maybe I'll start by describing a house, or a bad mood. Characters accumulate; emotions accumulate; events accumulate. Slowly, over time, the confusion comes into focus, and I have this set of characters and conflicts. I'll write 20, 30, 40, 50 drafts, and by the time I get to the 50th draft, I'll have a good idea of who the characters are and what their plight is. And then I'll write the next 30 drafts, and a story emerges.