As a result of the producers splitting the film into two parts, Screen Actors' Guild contracts now often feature what is called a "Salkind Clause," which requires producers to state up front how many films are being shot, and that the actors involved must be paid for each. The latter clause applies even, or even especially, when producers make that decision during or after production.
Various sources including Charlton Heston's memoirs say that Heston was offered one of the Musketeers - the Oliver Reed or Frank Finlay part - but Heston passed on this role or roles because it was too demanding - he had just done a run of action roles - and also because the part was a bit small, so he chose to play a cameo.
Originally conceived as a vehicle for The Beatles by veteran Beatle director Richard Lester. The filmmakers decided against this because they did not want the Beatles personalities overshadowing the characters.
Constance Bonacieux, played in this film by Raquel Welch, is constantly described in the original novel as "intelligent", whereas in this film she is played primarily for comic effect as somewhat clumsy and accident-prone and slightly less intelligent, though still fiercely loyal to the Queen.
The stunt people were terrified of Oliver Reed, due to his sheer ferocity when it came to fight scenes. He would often leap in without rehearsing. It got to the point where they would draw lots to see who would face him. Christopher Lee recalled, "I remember during a fight scene he came at me with both hands on the sword, like an axe, and I parried it and stopped totally. I said, "I think we'd better get the routine right". Then I said to Oliver, "Do you remember who taught you how to use a sword?" He said, "You did". And I said, don't you forget it". You see, I made The Devil-Ship Pirates (1964) with him for Hammer and he was a bit of a menace in that, quite frankly. People leapt out of the way when he had a fight, because he went at it absolutely flat out".
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 dived 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".
Raquel Welch and Faye Dunaway worked with trainers to make their big fight scene "as physical and brutal as Lester wanted," recalls Dunaway. The rehearsals went fine, but when Dunaway pushed Welch for real, Welch lost her footing, fell and sprained her wrist.
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. 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.
Richard Lester liked to film rehearsals and have a camera running continuously to capture anything unscripted that might happen. Lester also shot with multiple cameras (sometimes up to five) on one take, rather than in typical single camera style. So instead of using stand-ins for the long shots and moving in for a close-up of the star, Lester would film the entire scene (from close-up to master shot) at the same time. So, stuntmen were used only when absolutely necessary. In his autobiography Accidentally on Purpose, Michael York recalls "leaping onto horses whose saddles were deliberately unfastened only to revolve instantly underneath amidst dust and prancing hooves."
The spring and summer weather in Spain was grueling because of the heat. On one day of shooting at Arnjuez (the scene where D'Artagnan is inducted into the Musketeers), the temperature reached 122 degrees Fahrenheit. A reported 7000 bottles of water were said to be consumed by the large cast and crew necessary for the elaborate scene.
The sets were closed and the European press didn't like it one bit. But Richard Lester was concerned about having his ideas stolen as four other adaptations of the book were also in production at the time (two Italian, one French and one animated).