Why is this character straight and not gay? Why is this one Indian and not Black? What would happen if the main character were a woman instead? What if we started the first scene in the middle of an argument? In the middle of a fist-fight? With a man masturbating? What if the first sentence let us know exactly where we were? What if it gave away nothing? What if we never quite learned what was going on? What if we did, but it turned out to be wrong?
Logicians and philosophers like to speak of "counterfactuals", the sort of thing SF&F readers know best through their Harry Turtledove tomes ("What if the South hadn't lost the Civil War?"). What Gabe's talking about with his post is how every piece of work has this inherently counterfactual flavor to it while it's still in progress — it's protean, and so it can in the end become anything at all.
Yes, I'm grappling with this right now — gee, how did you ever guess? — and not with Welcome to the Fold, which is 99% locked down, but rather its successor project, tentatively titled Perfect Skin. That project, a resurrection of something I wrote ages ago and never finished (and subsequently lost), is in precisely that stage of amorphousness. All there is at the center of it is two characters and the rough outlines of a near-future setting.
You want more? Yeah. So do I.
The trick with this sort of thing, I've found, is not to push anything, but to just keep tossing and testing as uninhibitedly as possible. I've already filled a document with several thousand words of toss-and-test free-associative drivel-babble, and not one word of it is canon. Nor does it have to be. It just has to be part of the process driving me forward to let things run through my fingers until they come together.