I'm assuming you mean that the book came first and the film second. If that's the case you get much more detail from a book that can't be seen on a film.
For example, "He stood silently in the forest. He could feel the damp earth under his feet. He inhaled deeply and the musty, earthy smell of a recent rain was slowly replace with the smell of a smokey fire, he heard the scream and ran to the top of the hill. Looking down he saw a house on fire, bandits running back and forth."
In a film, they would show you a person standing in a forest, maybe show some smoke, you'd hear the screams and see the person running up the hill and looking down on the house with bandits running. The film might even try and show you the there had been a rain.
So you miss a lot, you miss what author intended. Now I've seen a lot of films that was as good if not better than the book, but even then you miss the author's intent a lot of time.