Acting for Two: Frank Finlay (Porthos) also played O'Reilly, the jeweler tasked by Buckingham to reproduce to two missing diamond studs.
Actor-Inspired Element: Cardinal Richelieu says to d'Artagnan 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 Richard Lester to find a place to include it in the film.
Creator Killer: An unintentional example: During the filming of The Return of the Musketeers, Richard Lester's longtime collaborative partner Roy Kinnear fell off a horse and broke his pelvis. He died of a heart attack the next day. Lester grew so distraught that he quit making films, though he would make one last project, a concert film, for Paul McCartney two years following Return's release.
Executive Meddling: What was supposed to be a four-hour long epic became split into two films because the producers didn't think that people would want to see a film that long. Since the actors were only paid the rate for one film, most of them sued and won. (This also directly affected Superman and Superman II from the same producers, as they were also originally shot as one long production, though still separate films. Richard Lester replaced Richard Donner for Superman II.)
Richard Lester has said in interviews the producers cast Simon Ward and Raquel Welch.
Recycled Set: The Duke of Buckingham's billiard room and the long hall behind it at his English estate is a leftover set from Patton. It is also seen in The Four Musketeers, where Milady and Buckingham speak about American Indians and also serves as a set of rooms in The Return Of The Musketeers for a different character in France some 10 years after the action of the first two films.
Sequel Gap: 15 years between The Four Musketeers and Return of the Musketeers.
Written-In Absence: Richard Chamberlain quit The Return of the Musketeers because he was angry at the producers lack of reaction over the death of Roy Kinnear. This is why Chamberlain's role is so brief in the final film.