When I draft a story, I am almost always an underwriter.
I write less than I could; I write less than a reader might need to fully comprehend a character or setting. Then, when I go back and read over what I’ve read, I fill in some of the gaps. I add action or bits of description into sections of dialogue, I add description into action. At the same time I’m cutting, of course–I often tighten conversations two or three times–but mostly, I add. My zero drafts aren’t outlines, but they have some things in common with them.
Others are overwriters. To figure things out, they must write everything down. Every bit of the world they imagine, every action, they must put down, and figure out later if it’s needed or not. In the end, this works just as well, but it doesn’t seem to be my method.
Somebody out there has to be a “just right” writer, but I’ve never yet met anyone that would admit to it.