Oliver Reed was severely injured and almost died when he was stabbed in the throat during the windmill duel scene.
As a result of producers Alexander, Ilya, and Michael Salkind splitting this movie into two parts, the Screen Actors' Guild contracts now often feature what is called a "Salkind Clause", which requires producers to state upfront how many movies are being shot, and that the actors and actresses involved must be paid for each. The latter clause applies even, or even especially, when producers make that decision during or after production.
Michael York had his leg cut in one duel and almost lost an eye in another. Oliver Reed took a sword to the hand. Frank Finlay was struck in the face by a two-by-four, and burned in separate fight scenes. Sir Christopher Lee fared better than most of the cast, getting off with just a sprained knee and a pulled shoulder muscle. It got so bad that at one point, York remembers doubling for his injured stunt double. He later resorted to stuffing his script inside his clothes for protection.
The cast stayed at a plush hotel in Madrid. As a practical joke, Oliver Reed removed the goldfish from the ornamental pond in the dining room late at night, keeping them in his bath, and replaced them with fish-shaped carrots. The next morning at breakfast, he dove into the pool and began devouring the fake fish. The manager called the police and Reed was hauled off the premises bellowing, "You can't touch me! I'm one of the Musketeers!"
Frank Finlay played two parts. He also played O'Reilly, the jeweler tasked by the Duke of Buckingham (Simon Ward) to reproduce the two missing diamond studs.