I have a difficult time watching movies based upon books I’ve read because I’m almost always disappointed. When you go into a movie knowing the plot, sometimes you keep waiting for an event to happen or a character to appear—only it never happens. Or sometimes they rewrite the story in a way that tries to condense the plot or they add characters that don’t belong that confuse you into wondering if this movie is really based upon that book.
I expect some plot edits since a movie can only fit so much in its allotted time constraints, but one aspect that can be especially painful to watch are actors that are utterly miscast. They look weird or they sound wrong or perhaps they just can’t act; basically, they are nowhere near the vision you had of the characters while reading the book. So every time miscast actors speak or show up in a scene you can't help but roll your eyes or sigh is distaste because it's so disappointing to watch characters you loved so much in your mind, come to life in such an unappealing way.
No, watching a movie based upon a book you like is quite the difficult task because you have expectations that will unlikely be met. This is why I prefer to watch a movie in pure ignorance of the book; then I have a much better chance of enjoying the movie. And there are quite a few movies based on books that I like a lot. But whether I read the book before or after I watch the movie, a vast majority of the time if I had to choose which was better, I would choose the book because the book often has details and depth that cannot be translated in its entirety into a movie.