As I write “A Bridge To The Quiet Planet” I keep discovering more about it. Perhaps I find a theme that I missed or that should be incorporated into the story. Or I may realize I missed something and that I have rewriting to do. Or something just works better. ...
Writing is really a method of discovery. So you have to write in order to know what you’re writing. In turn, you constantly find out more.
The way I've put this on my own is, you have to get boots on the ground with a story, because there are things there that simply won't be visible at 30,000 feet or from a satellite view.
The back-and-forth between characters, good and bad. The in-the-moment decisions they make that not only let them express themselves but drive the story. The little things that become big things and that inform the little things. The execution.
I've previously posited a thought experiment where you give the same story outline to ten different people. You can make this outline remarkably detailed, down to specific scenes and beats and whatnot. Ask each of them to produce a story based on this skeleton. Rarely, if ever, will two writers produce the same story — or, even if they do, the flavor and the tenor of those stories will be impossible to mistake for each other.
One of the contributing reasons for this, I suspect, is the way no two writers will have the same way of navigating the boots-on-the-ground experience of writing the same story. I sometimes wonder what kind of story Steven would have produced if I had given him the raw materials for one of my other works. (Just to be ornery, I might make it something like Welcome To The Fold; god knows what Steve would do with that Gordian knot of thorns.)
The only time you ever really get to know a story is when you're between one sentence and the next.