I understand what you mean about reading the book before the seeing the movie. I can't remember what John Grishim book I read, but in the book I pictured one of the characters to look and act like Peg Bundy in the TV sitcom "Married with Children." In the movie they cast Holly Hunter in the roll. What a difference it made. She just didn't fit the part and I really had a hard time watching the movie.
After that, I now read books AFTER I see the movie. That way I am not disappointed by the results. This works much better.