D'Artagnan is saved from beheading by the other Musketeers, who disguise themselves as a priest and headsman. While this scene is not to be found in the original novel The Three Musketeers, there is a similar scene in the sequel, Twenty Years After, in which the four musketeers attempt to save England's King Charles I by disguising themselves as men working on the execution scaffold.
Athos calls Countess D'Winter "Sabine". In the original novel, her character is known by several names (Milady de Winter, Anne de Breuil, Comtesse de la Fère, Lady Clarik, Charlotte Backson, or simply "Milady") but Sabine is not one of them.
Cary Elwes was also considered for one of the roles until well into the casting process, and appeared in a pre-production interview from New York's Planet Hollywood with Charlie Sheen and Kiefer Sutherland that was broadcast on E! Network.
The throwing weapon with cords and weights that Porthos uses to catch the guard during the fight at the convent is real. It is called "bolas" or "boleadora" and was used by Patagonian gauchos, in modern day Argentina. As for the time of the film these weren't known in Europe, this could support Porthos' claims of his many travels and adventures, despite generally being considered as fabricated.
A rival TriStar version was also in development at the same time as this film, with star Johnny Depp and director Jeremiah S. Chechik attached. Ultimately, it fell through. Oliver Platt had also been approached to play Porthos in that version as well. Interestingly, Chechik would direct Platt two years later in the film Tall Tale (1995).
The dungeon scenes were filmed at Seegrotte, Austria: an old mine which was flooded early in 1900's due to an underground water source and was later used by the Nazi's during the second war to conduct military research. The site is now open to the public and famous for its underground lake. The scene decoration of one of the prison cells, as well as the dragon-head boat are still kept intact at Seegrotte and could be visited.
When Charlie Sheen's character Aramis says "See you in Calais", it is a reference to the famous General George S. Patton Jr. quote yelled at Dwight D. Eisenhower out of Patton's hotel window - three hundred years after the film is set.
During the final duel with Rochefort D'Artagnan leaps over him, and spins, landing on the other side of him to avoid a blow. Rochefort, all decked in black, says, powerfully, "Impressive!" Mirroring, in homage, Darth Vader's fight with Luke Skywalker in Star Wars: Episode V - The Empire Strikes Back (1980).
During a bar scene, D'Artagnan (Chris O'Donnell) asks Aramis (Charlie Sheen) why Athos (Kiefer Sutherland) is drinking alone. Aramis replied "Because he takes his drinking seriously". This is somewhat ironic, given Sutherland's many arrests for DUI.
Some sequences were shot in Cornwall, England. A small woods called Golitha Falls was used in one sequence when the musketeers are being pursued by guards. The small harbour village of Charlestown is home to the galleon that was used in a night-shoot. Scenes were also shot on Pentire Head, Boconnoc Estate, Doyden Castle and in the grounds at Llanhydrock House