The differences between a book and a movie take place for many different reasons. Whereas a book must describe everything in detail so that the reader can imagine it, the movie has the benefit of video to show in a flash what a book may take several paragraphs or pages to show.
Many people are disappointed because the characters in the movie do not look like those the reader has imagined. Also, some movies are not true to the book and may change the color or ethnicity of a character. It may be fine for those only watching the movie but it typically annoys the person that has also read the book.
The hardest thing to translate is a person’s thoughts or motives. Thoughts cannot be shown on the silver screen. The director must turn those thoughts into action including tone and inflection of the voice as well as facial expressions.
In more “literary” works the person that writes the screenplay must drastically reduce scenes, characters and settings that are not germane to the overall story. While this may change the direction of the story it is often seen as being a necessary evil.
Those that think they can just watch the movie instead of reading the book are typically going to be in hot water as the two are rarely the same.