Any time that a book is adapted to film, there are changes. It is bound to happen and it is the nature of adaptations. There are two different kinds of people that go to see those kinds of movies, those who have read the book and those who have not. If you are wanting to be counted among the intellectuals of the world, then you will want to be a part of the first group.
When you read a book before seeing the screen adaptation of it, you have already formed in your mind how you perceive it to be. You create the images of the characters and the places based on your own life experiences. After you see the movie, you can never go back to a blank canvas. You will always have the director’s interpretation in your mind as how it is supposed to be. Forming your own opinion on the book after that is difficult, though not completely impossible.
You also sound extremely intellectual when you talk about the movie afterwards. You can use your knowledge of the back story behind the script or of events that transpired to form your own opinions about the director’s artistic choices. You understand more of why the film is presented in the way that it is if you have read the book. It also helps because you can change your opinion and you can like or dislike what the director chose to do.