Concept cover by Glass Knife Press.
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)
E. P. Beaumont 
Oh, yes, the Marriage Law Prompt and the Hollow Man prompt resulted in some truly excellent stories in Snape-Granger fandom and what’s more, reseeded itself into other fandoms, too, though I didn’t see much of those. There are often lots of other prompts and I generally tell people, ‘I have over 50 Harry Potter plot bunnies already, I don’t actually need any more…’
Unpack: The Marriage Law Prompt assumed that Wizarding numbers were way down after Voldie War II, and that the Ministry needed to take action to fix it. This resulted in various scenarios where a) the purebloods attempted to neutralize important Muggleborn such as Hermione by forcing her to marry someone in their circle (often a random Malfoy, and a lot of those were written before it was known that Snape was a halfblood), b) Hermione must marry Snape to keep him from execution or being Kissed, c) Hermione must marry someone of age, and Ron isn’t, yet, and d) random drawing, often fixed by Umbridge just to be cruel since the alternative to Hermione is having her wand snapped.
The Hollow Man prompt presumes that Snape survives, but under mind control as some sort of helpless slave, and he must be restored somehow to full functioning (won’t note the most popular scenarios, don’t you people have dirty minds yourself? ).
Excellent points here – fanfiction is by definition profoundly intertextual (as the scholars would say). Prompts generate bodies of stories, which themselves incorporate conversations about characters, motives, social assumptions. Once a “Usual Story” comes to life, it then gives birth to counterfactual and parodic offspring.
In short: stories breed more stories. Like … plot bunnies, in the bunny hutch.