As a multi-fandomer, it's not as easy as you may think. At least not for me. Um, I guess I can multi-task and keep fandoms separate in the sense that if I'm watching something, I'm really absorbing the characters first, and then the storlyine and plot second, but it's the characters I get a feel for first, and my connection to them is instantaneous because it's fresh. I suppose I just pick up on things, the idiosyncrasies of the character, and I'm more in-tuned with them early on? I don't know, but I don't write LONG epic stuff from a few episodes of a fandom.

The only fandoms I've really been able to do that is Heroes and Inuyasha. That's mainly because I've seen more of their episodes numerous times, and I've been way more engrossed, paid way more attention, than others.

The process though for writing them for me is tuning into a particular scene, remembering it vividly, through facial expressions and dialogue, and then use that as a foundation for expanding upon that for something else. If the fandom or episode is fresh in my mind, it's easier for me to do that. After awhile, the characterizations sort of stick and become a baseline.

I admit some fandoms that I've lost track of (Avatar, Harry Potter, Twilight), I do have to revert back to the source material, watch an episode again or reread a passage. It's the same as research, and sometimes I don't have to do that, I can just go on Wiki and look it up and be reminded of the scene.
It takes a little work, but it's worth it. :D