The movie and the book
The novel was written by William Bradford Huie, the screenplay by Paddy Chayefsky. Some viewers may be interested in the differences.
Commander Madison's attitudes about building statues honoring dead war heroes is all Chayefsky. Huie's Madison (named James) has nothing to say on the subject. In fact, the word "coward" appears nowhere in the novel, and the only use of "cowardice" is out of the lips of Madison, when he chews out a fellow officer on D-Day.
The wonderful scene in which Joyce Grenfell and James Garner discuss the morality of war over tea is pure Chayefsky. In fact, Emily Barham's mother never even appears in the novel!!
The Admiral (Melvin Douglas) doesn't crack up in the book, nor does he dream up the "first dead man on Omaha Beach is a sailor" line. He simply directs that the D-Day invasion be filmed by Madison, and Madison complies.
Bus (James Coburn) does not land on the beach and does not shoot Madison. Reports of Madison's death wouldn't make sense in the novel, which is told in the first person.
The one-day delay in the invasion ("the moon didn't come out") plays no part in the novel, though it is a pivotal point in the movie.
In short, the movie, one of my favorites and a classic, owes its quality to Chayefsky, a solid cast (except for Coburn, whose reactions are conspicuously uneven), and a good first-time director, Arthur Hiller.
Finally, according to Hiller, the Madison role was to have gone to William Holden, with James Garner playing Bus. When William Wyler dropped out as director, Holden quit the project.