1. Frodo: Whatever it was, I certainly did NOT imagine him with those huge blue eyes and the dark hair. I imagined him to have sandy coloured hair (it's mentioned he has fairer hair than most), with brown eyes, since Tolkien never really gave us a description of him, but he gave a general description of hobbits.
Sam: Brown hair, brown eyes. Apparel very much like the Ralph Bakshi adaptation. And I expected him to have darker skin in the movies, since Tolkien mentions at least twice that Sam has brown skin.
Pippin and Merry: I imagined these two to be very much alike, though I admit I thought Pippin would have lighter hair.
Legolas: I think this guy was the clearest out of all the characters I had in mind, along with Gimli. I imagined him to have shoulder-length wavy blonde hair and dark blue eyes, wearing green and brown, just like in the books.
Gimli: I believe John-Rhys Davies hit it on the spot here. I imagined Gimli to look EXACTLY as he did in the movies. I only felt he should have looked a bit more menacing. The Gimli in the movie was a shameful, insulting charicature.
Gandalf: Again, Ian McKellen nailed it. I just imagined a sharper nose, and blacker eyes.
Strider: Very foggy. Most vague character I came across.
2. Yeah, changed a lot. But I think Boromir and Faramir looked quite good. I actually liked the look of Boromir better in the movies than in the books. Aragorn looked good, but somehow he didn't look authentic enough for my taste. I'm going to deviate a bit from here by saying that PJ really killed some of the characters. The movie was awesome, but Frodo looked and acted like a pansy, Arwen was TOTALLY out of character, Pippin, Merry and Gimli just made me laugh, Legolas practically had no role, Gandalf looked far too weak, and Aragorn was just a total Gary Sue.
3. Yeah, actually. I designed some clothes and faces in a little black notebook that I keep exclusively for LotR
4. Yeah, definitely. Alan Lee and John Howe. I think they really nailed the feel of the books. The painting of Legolas and Gimli at Helm's Deep by John Howe really influenced my thinking. So did the painting of Gandalf walking with his staff (John Howe again). The Silmarillion characters were always a bit hazy for me, so looking at illustrations by Alan Lee helped. Turin looked EXACTLY how I thought he would
5. I've written some fanfiction where the characters are totally true to their appearances and characters in the book.
Cheers!