When I read fanfiction, I usually picture my initial image of Edward added with whatever images the author throws in. Sometimes he ends up looking more like Rob than my version of Ed, and sometimes he's just different altogether. I guess it depends on the story. As for the other characters... that also depends on the story and the character. Jacob, for example... in most stories I can imagine Taylor, but there are some things that he doesn't seem to fit the character that I'm reading (or writing), so I either imagine my initial thought of post-transformation Jake or go with whatever the author has described. I think most of the Cullens, aside from Alice, I imagine the movie people becuase they were pretty much what I had thought of in the first place. Alice is much smaller than the movie version when I read fanfic. Yeah... mostly the movie people unless it's a main character like Bella, Jake or Edward. Bella's just... she's usually a lot more 'everyday' than Kristen when I read or write. If that makes sense.