Peter Cushing did double duty on this film. In additional to his own role, he would feed Louis Hayward the lines for the split screen shots. Director 'James Whale' initially cast him only to play opposite Hayward in the sequences where both twins appear together, but was impressed enough with the newcomer that he offered Cushing a small part on horseback. This was Peter Cushing's film debut, and he had the unique opportunity to view his own rushes and improve his own performance, especially since none of it would be used in the finished feature.
In 1952 'Alan Hale Jr.' played Porthos Jr. in a quasi-sequel, At Sword's Point and recreated his father's role in the remake The Fifth Musketeer.
Nearly all of the characters in this film actually existed, but none of the characters who die in it actually died that way in real life.