The book is more full, naturally, but I'm not a purist - the movie tells the same story using the same characters, and the differences are not much more than allowable variations, considering the limited time available. Now go read the book, and you'll have a ready-made 'cast' in your head. Or, if you see other actors doing a better job, cast it your way at storycasting.com, the premier 'fantasy casting' website.
My rules for a book adaptation: the movie has to tell the same story in the same way using the same characters in the same settings, without adding any junk or leaving out a major piece of the story - and still be just as enjoyable as the book. This movie meets all those, and was well acted. Robert Pattinson perfectly captured the smoldering intensity of Edward, and Kristin Stewart, while not as annoyingly self-absorbed as the book's Bella, was still the distant, non-communicative, secretive teen. Great effects - just enough - and all set up for the next movie. Bravo!