I saw the movie first before reading the book. I find it interesting how some things were condensed, moved around, or combined to make the story move faster. The book contains many more enjoyable details. One big difference (for me) was the greater depth given to the Cullens, so that you care more about them and understand their motives (I really wish the movie had included Edward's revelation to Bella that Rosalie seems hostile because she is jealous and not totally at peace about being a vampire). The introduction of James, Victoria, and Laurent earlier in the story is also a large change(we see some of their hunting activity, and a character is created in Forks that gets killed by them, I guess to make it more personal for the viewer--we want to be horrified by these newcomers and feel threatened by them, not think "cool, more vampires!" Jacob Black is also introduced earlier, and look for some changes in his dialogue with Bella, which changes how early Bella suspects Edward's true identity.
For the most part, the movie is very true to the book, even when it changes things. For example, the fight scene in the ballet school is only hinted at in the book (Bella was going unconscious, so she didn't see most of it), but nothing is portrayed that changes the story or alters motives or intentions.
For the most part, the movie is very true to the book, even when it changes things. For example, the fight scene in the ballet school is only hinted at in the book (Bella was going unconscious, so she didn't see most of it), but nothing is portrayed that changes the story or alters motives or intentions.