"Well, isn't what you're writing just fan fiction, then?"
It was the "just" that made me realize I was going to have to answer carefully, that I couldn't just say, "yes" and move on.
The thing about fan fiction is it has a bad reputation. The phrase can conjure up images of pervy people taking favorite characters out of beloved books and writing dirty dirty sex scenes for the amusement of the other pervy people on the internet. And no, I'm not going to play naive, and pretend like that never happens. But the idea of fan fiction is so much bigger than slash featuring Harry and Draco.
I got asked the question because Linger contains elements of and characters from the Arthurian mythos. I'm not doing a straight up retelling of the story (and incidentally, why are retellings never labeled as fanfic? Is it because they got published?) but it's a strong presence in the text, and I had made no secret of the fact that the King Arthur story is one of my very favorites. I wrote what I did because there were places in the pre-existing and agreed upon story (places dealing with Guinevere, specifically) that didn't satisfy me as they were written. There was still room for a story there, and so I wrote my version of it.
And yes, if fan fiction is writing using someone else's characters, that's exactly what I wrote. The novel I'm currently writing could be seen that way, too, since I'm using part of the Tam Lin ballad as fuel for my story.
Especially if you reduce things to the bare skeleton of plot, there are only so many stories. The thing that makes a story beautiful and unique is the way in which it is told. I think people wanting to spend time in a universe that a writer has imagined, wanting a deeper relationship to the characters she has written is a compliment to the writer and to the story.
So yes. What I wrote is fan fiction. I think that's just fine.