All writing is improvisation.
This cannot be repeated often enough. As a fanfic writer I alternate between writing stuff that just randomly popped up to tug my sleeve, and writing to prompts that appeal to me. There’s a whole very active scene in fandom where you propose a prompt and people write to it.
You get multiple stories, all very different, from the same prompt.
A plot is an elaborate prompt. Sometimes you build that prompt in great detail, with timelines of events and who’s where when. Oddly enough, that structure gives huge scope for improvisation; you have safe knowledge of the background, the stage set as it were, so there are things already in place that you don’t have to worry about while you let the characters come into the room and interact with each other. You already know the details so you don’t have to fuss with them.
Other writers like to just jump in and write into the darkness, having only the faintest idea where they’re headed, except in the sense that X, Y, or Z juicy event is waiting for them.
Sometimes all you have is a draft final scene you’re aiming for. That distant beacon gives your efforts some forward momentum, but the stuff that comes up as characters interact, make decisions, do things, will sometimes carry you wildly off course from what you thought was your real destination.
That’s fine. You can throw stuff out.
Excerpt from forthcoming From Fanfiction to Original Fiction (Vera Rozalsky with E. P. Beaumont)