The characters - in fact, I think the only way to create a good story is starting with the characters. Though there is one aspect of the plot, which might be there before the characters: The ending. I always knew exactly where I want a story to end (after all, one needs a basic idea, otherwise you have no idea what kind of characters you need).

Fanfiction are naturally a special case, because the characters are already there...but when you write one, you naturally chose a scenario which fits the characters, not the other way around.

I think, the basic idea is the egg, the characters are the sperm to give the idea the live, and what's growing of this is the plot. I don't know how often I had to change a plot I had in mind, because my characters decided to go into another direction. The fight Neal and Peter had at the end of "ConCurrent" is an example for that. That one wasn't planned at all, I just wanted to wrap up the story, but Neal and Peter being who they are, it just made sense to add the fight there. It practically wrote itself.