A movie and a book are vastly different experiences. A book, and here I’m going to assume that most of the time the word “book” is being used to mean “novel”, is primarily an internal, cognitive experience. Books are experience through language. Books are interpreted by the reader. Meaning is created within the reader’s mind. Books, the best ones anyway, require as much input from the reader as from the writer. Books are the ideal medium for letting a story find its duration, for complex, interwoven storylines and truly deep, complex characters, settings, and plots. Books allow one to take time to focus on insignificant details that don’t push forward the plot, but enhance the flavor of the story, add nuance and subtlety. Books take time, are meant to be lived with and in for a while.
A movie (and here I’m interpreting the word “movie” to mean primarily a feature length fiction film) is more of an emotional experience, which happens to us rather than within us. Movies are primarily visual experiences, with sound playing a large, though smaller role. Movies have their own visual and aural language, and are “read” using the visual and auditory parts of the mind rather than the language part. For most movies, even very good ones, the viewer is a more of a receiver of the content than a co-creator of it. Movies have a very narrow acceptable duration (the vast majority are 90-150 minutes, with very few exceptions), and are almost always intended to be absorbed in their entirity in a single sitting.
Enough philosiphizing. Now for some practical matters. It is the time difference that creates the most problems when adapting a novel into a movie. Of Mice and Men, about 120 pages in most printings, becomes a 2 hour plus movie when adapted faithfully. Holes, a roughly 160 page children’s book, leaves out some parts of the story and still has to rush from plot point to plot point. Start with a novel that is substantially longer, and it becomes necessary to do some major condensing–leaving out or compositing characters, leaving out or condensing plot points, and reducing the number of rounded characters. No filmmaker is ever going to do this in a way that makes all of the readers of the book happy–some favorite parts have to be left out just because there’s no time for them. Also, some things that work in the abstract, or work just because the author is skilled with the language used to present them, just don’t work in a visual medium.
Movies and printed words are fundamentally different media for presenting a story. Movies should be compared only to other movies and books only to other books. No movie, no matter how well or badly made, has ever altered the quality of the book from which it was adapted.