I've been doing a lot of thinking about the history and culture of a whole new world for Immortal Destiny, and come to be astonished anew at how incredibly organic the process is. There's a certain feeling of inevitability about it. Once the world is set in motion, you switch from creating it to reporting on it, how it is, how it has to be.
It was this way with Perplex City, too. There's an initial seed of creativity - say, creating a handful of celebrities and their personalities, or describing an organization and its interaction with government. And then you begin to map out their interactions with one another. As time goes on, these situations and their outcomes become so clear, so obvious, so true, that it becomes awkward to say "Yes, I made this." Because the characters and the world together did it, not you. You're just explaining the only thing that could possibly have happened, given that small seed of actual creation you did in the beginning.
Sometimes you have to plant another seed or two, to keep the world from hitting equilibrium. A static world is a boring one. But mostly you're just reaping the harvest of those small, tiny ideas you began with, and how they grow and change over time in ways you never would have dreamed when you first began.
I used to think that writers who talked about their characters having their own ideas were crazy. I used to think that writers who described how their scenes never turned out the way they'd expected were crazy. I used to think that writers who talked about their worlds being real were crazy.
And now it happens to me, too.