For a commissioned story, me and the editor have already agreed on a fairly detailed outline, and I stick pretty closely to that (if I remember to keep checking it - otherwise I go wandering off and get hopelessly tangled up!). But there's still a huge element of improvisation in putting the flesh on those bones. I threw in a bit of tech at the start of Red-Handed which ended up being instrumental in the story's climax and its resolution, and that tech isn't even mentioned in the outline!