Some very interesting replies to my post - thanks everyone. Just to clarify a little what I was getting at was whether we react to characters fleshed out for us on our screens any differently to characters we encounter in the pages of a book. I also wonder whether part of the motivation for writing tv fanfic is to recreate or hold on to that character as performed by a specific actor with all their mannerisms, physical characteristics etc. With a book we each imagine what Mr Rochester might look/sound like etc. When we've seen Orson Welles in the role it's more difficult not to imagine the character as such, even if we don't particularly agree with the casting/interpretation. With tv shows and film, as well as the characters, we also have sounds (music), colours and visual imagery that make the show what it is and sometimes this may be reflected in fanfic. What originally fascinated me was the fact that people do want to write stories and 'just' rely on text - I haven't come across any fanfic which uses sounds, colours, images, hyperlinks etc. I have seen fanvideos on youtube and fanart, but I was fascinated by the impulse to 'write down' the stories. Perhaps this gives us an opportunity to enter the characters' thoughts, to slow things down so we can figure out and debate what has happened, and maybe also to give us a sense of ownership and participation in the fandom?
Would love to hear more from you, but thanks for all your input so far!