Because, uhm, let's say Character A and Character B existed some time in the late 1800s to early 1900s. Their story was fascinating (at least, the way sources make it out to be), and there was a lot of tension* between them and they were both really interesting people and all that. I'd like to write fan-fiction about them, but I don't want to cheapen their places on history with LAWL FANDOM. I feel I'm pretty informed, but I do not want to romanticize anything and want to stay with what supposedly happened. But at the same time, I want the story to be fun to read.
I guess what I'm asking is, what can I get away with before it becomes defamation of character? I'm not speaking in legal terms; I hugely admire Character A and Character B within their historical contexts, but at the same time I have silly thoughts about slash/crack-fic/whatever.
*Even worse, when I wear my slash goggles it turns into SEXUAL TENSION. Yes, I am pathetic, especially considering Character A was celibate and pretty erotophobic and Character B had, like, a bajillion children and hated Character A with intense, fiery passion. BUT OTHER THAN THAT, IT COULD TOTALLY WORK. TOTALLY.