OK - here are the main differences between the 1964 novel and the movie.
As I suspected, the movie follows the book pretty closely - scene-for-scene mostly - but the body count is significantly higher with several extra action scenes and the ending is slightly different. In the book, Sturges doesn’t kill any guards or posse members or the sheriff so it is easy to acquit him and give him a pardon. The Italians are less bothered by such niceities so Sturges kills loads of law officers and prison guards in the movie and the movie still ends with him expecting a pardon.
Massive SPOILERS to follow
- Opening shootout in which bad guys steal horses and kill all the ranch hands not in novel (the relationship between this theft and the train robbery, if any, remains as unclear in the novel as it is in the film). Stolen horses referred to in dialogue but no mention of any shootout to get them. Novel opens on nighttime campfire scene in which Maynor appears.
- Like the film, the book cuts from the scene in which the sheriff beats-up Roy directly to Yuma jail (so no trial sequence etc). In the prison break, Sturges escapes without killing anyone. The body count is much much lower and there is no massed shootout between guards and prisoners which goes on for ages
- Sturges doesn’t kill Savage either. Savage is killed by Encarnation during the fight in her house. She is killed also in the fight.
- Sturges doesn’t kill any pursuing posse members. Instead, he escapes Yuma by hiding on a ferry rather than horseback.
- Sturges doesn’t kill any of the Mexican bounty hunters. He lets them take Mason (who’s called something else) back to Yuma without intervening.
- There is no cantina brawl.
- Sturges kills Baldy in a fight in Baldy’s shack. In the film, Maynor kills Baldy.
- The ending is a bit different; Sturges finds the hidden loot after a long search of the way station (rather than Maynor digging it up) but then the Sheriff appears and is killed by Maynor and Sturges kills Maynor. The sheriff is not in league with Maynor like he is in the movie and Sturges does not shoot the sheriff or loads of deputies. The chief railroad investigator then turns up with a posse and says Sturges is innocent of the robbery and since he has only killed a few bad guys since release he is OK for a pardon.