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 the rushes and improve his own performance, especially since none of it would be used in the finished feature. As 'Second Officer,' he can be seen 17 minutes in, with two lines of dialogue: "How could I mistake it. I've been here before".
Nearly all of the characters in this film really existed, but none of the characters who die in it died that way in real life.
In Alexandre Dumas pere's novel, The Vicomte de Bragelonne, Philipppe is substituted for Louis XIV for only a short time before regaining the throne and re-imprisoning his brother. In this film, Philippe is successfully made a permanent substitute. Subsequent adaptations of the book have more closely followed this film than the original novel, sometimes even crediting this film's screenplay as their basis as well.
Fouquet's statement that "we must all hang together, or we shall assuredly hang separately" is thought to have been spoken by Ben Franklin at the signing of the Declaration of Independence.
Nigel De Brulier previously played Cardinal Richelieu in The Three Musketeers (1921), The Iron Mask (1929) and The Three Musketeers (1935).