The film’s screenwriter Christopher Hampton describes the structure of the film:
Atonement is really a two-act structure, with the two acts pretty much equal in length – or, if anything, the first act a little longer than the second – which means that you have to manage a tremendous gear shift, from country house life in 1935 to the retreat from Dunkirk. I thought the best way to do that was the way The Deer Hunter did it, where you just go – bam! – from one world to another. I think The Deer Hunter is a more than dubious film, but it does have this brilliant effect in it: an extreme change of direction which galvanises the audience so that they trust you to bring the whole thing together again at the end. You also have to thread into those two acts the very short last section of the book, which reveals that it’s been written by this character called Briony, who’s a thirteen-year-old when the story starts, and that she wrote it to atone for this crime which she inadvertently committed as a girl… Although this last section of the book is a real revelation, there are discussions and observations throughout about the nature of fiction which make it retrospectively logical, and you need to reproduce that so the ending doesn’t just come out of nowhere. So that’s the other technical difficulty… Ian McEwan has read every draft, and been rather assiduous. The revelation at the very end is the most complicated bit to work out in film terms. But I think we have.