Cardinal Richelieu (Charlton Heston) says to d'Artagnan (Michael York) that he has no personal enemies, only enemies of France. This line is in neither the original novel nor the script. Charlton Heston came across this quote of the Cardinal's when researching the role, and asked director Richard Lester to find a place to include it in this movie.
Producers Alexander and Ilya Salkind were sued by the actors and actresses who claimed they were tricked into thinking this movie was to be part of Les trois mousquetaires (1973). They won their case in court, but did not receive as much money as they would have if they were paid separately for both movies.
This movie's Oscar nomination for Best Costume Design marked the first time that a sequel received a nomination when its predecessor was overlooked. Furthermore, when it came to the BAFTA Awards, Yvonne Blake was solely nominated for Best Costume Design but when it came to the Oscars, both Yvonne Blake and Ron Talsky were nominated although Talsky only designed the gowns for Raquel Welch's character.
According to his autobiography, "Lord of Misrule," the late Sir Christopher Lee, who played "Rochefort", explains how angry the rest of the cast were in receiving less pay in the whole ordeal as opposed to what they would have received had this been a separate production. Lee, on the other hand had no problem with the pay he received, having grown accustomed to meager salaries from his work in numerous independent films beforehand.
The actual on-screen title is: "The Four Musketeers (The Revenge of Milady)".