1). If you read the books before the movies, what (if anything) did the characters look like to you?
I rarely create mental pictures of characters. Sometimes, if an author is very specific in their description, I'll make a basic Barbie and stick the proper features on it, but never anything more personalized.
2). After the movies came out, did this change?
Yeah, I borrow images from movies all the time. LotR, Narnia, pretty much everything. It's a bit of a lazy tick.
3). If your answer to 2 is yes, have you tried to recover your previous mental pictures and/or create new ones in response to the films? If so, how?
Nah. That sounds quite a bit like work.
4). Have any of your mental associations with characters been otherwise influenced by other sources (i.e. illustrations, fanart, other adaptations)?
Some of the Sil characters, definately.
5). Have you ever engaged in any sort of sub-sub-creative effort that draws upon any of these mental pictures (i.e. RPGs, fan fiction, fanart)?
Oh, yes. I most write parodies, so my chars end up being movie-verse plus some. My Frodos are even more pathetic, my Pippins are even more clueless, my Legolases are even more "perfect", etc. So I take the profiles given to the other chars and I make them "even more so".
Fan-art, for me, focuses more on obscure Sil chars than LotR chars, so the movies don't really have an effect.