I'm big on reading the book before seeing the movie. I like to have my own version of the story in my head before the look is dictated to me by a movie. Unfortunately, I end up spending the entire time watching the movie grousing about how they changed it and that's not how it was in the book. Usually the book is better, but sometimes I'm surprised and the movie has been as good if not better than the book.
Often times, I've read a book just because I heard that they're going to make a movie out of it.
Other times, I see a movie and hear that it was a book first, so I read that book and try to block out what I saw in the movie, so I can form my own vision. It doesn't always work that way, but I try.
I read the Harry Potter books before I saw the movies, but the movies had already been released, so I already had a vision of what Hermione and Ron looked like, even though they're described differently in the book, so now I have two versions of what the Harry Potter kids look like in my head.
I'm such a visual person, that when I read a book, I always let the story flow in my head like a movie. When I see the movie, I expected that same movie to play out, but that is not always the case. I have had to come to the mindset that movies are merely adaptations and can only be a synopsis of the book, and sometimes they have to add in things in order to make the story flow better as a movie. It drives me crazy, but I have to talk myself down; telling myself that this is only an interpretation of the book, another version. I still yell in my head "That's not how they did it in the book!" The funny thing is, if I see the movie first and read the book second, I never yell in my head "That's not how they did it in the movie!" I think well, they sure got that part wrong in the movie! I always blame the movie.