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. As 'Second Officer,' he can be seen 17 minutes in, with two lines of dialogue ("I've been here before").
In 'Alexandre Dumas pere''s novel, The Vicomte de Bragelonne, Philip is substituted for Louis XIV for only a short time before regaining the throne and re-imprisoning his brother. In this film, Philip 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.