I started writing fan fiction for the CRPGs I play, mostly because the writers for the games absolutely suck at writing relationships. (Yes, David Gaider, I am talking to you.) Honestly, even with a ratings-required fade to black, the older games managed a great deal of heavy innuendo that the later games lacked. Well, until Dragon Age, thank the Maker!
Then I sort of slipped into a bit of LotR fan fiction because I like elves, particularly Tolkien’s not-at-all-fluffy elves. But I will say, while I would never speak ill of Lee Pace’s portrayal of Thranduil Oropherion, I write based on the books and not the movies or actors.
I have to really geek about a fandom to want to write for it.