I watched the movie that was made from this book before I read it, which is not my usual MO. Given my familiarity, then, with the outline of the story, I was almost shocked to discover how comparatively briefly the movie had covered the introductory portion of the book, set in 1935, which sets in motion the entire plot. That portion of the story takes up nearly half the book, and less than 1/4 of the movie. Admittedly, there is almost no way to render the depths of Briony's thoughts without a tedious voiceover, but more attention to that aspect might have made her actions more understandable, and made her more of the primary character she should have been. Hollywood chose to focus more on the romance than the betrayal and attempt at atonement - no surprise there.
Although I thought the movie was fairly well done, as usual it did not do justice to the novel. How could it? McEwan's use of language is so gorgeous, it's impossible to translate it completely into the film.