It's a given that the same actor will play Rudolph and Rassendyll.
Ronald Colman in the lead role in the 1937 film. This was one of Ron's specialties; he'd been playing dual roles of one sort or another since his silent film days.
Creator Backlash: David O Selznick was unhappy with the 1937 version, especially the action scenes.
Playing Against Type: As noted below, Douglas Fairbanks Jr plays the villainous Rupert. He had wanted to play both the leads instead.
Remake Cameo: Lewis Stone played the leads in the silent version, and played the Cardinal in the 1952 remake.
Shot-for-Shot Remake: The 1952 version even used the same screenplay and most of the camera angles are the same. There are a few differences however.
What Could Have Been: Fay Wray screen tested for Flavia in the 1937 film. Douglas Fairbanks Jr held onto the clips of this and they survive in the Motion Picture Academy's archive.
Two stars of the 1937 version, David Niven and Douglas Fairbanks Jr., took their swashbuckling to Real Life in World War II. Both served in elite units (Niven in the Commandos, Fairbanks in the U.S. Navy Beach Jumpers) both saw combat and both were highly decorated for their actions.
By this point in his career, Ronald Colman had played so many double roles that he was fed up with them (and had, in fact, refused to do so for A Tale of Two Cities). On the other hand, the King's role is tiny and Rudolf Rassendyll gets most of the action.
Reportedly Douglas Fairbanks, Jr. was extremely reluctant to play a villain, but after he allowed himself to be talked into it he turned in a bravura performance that literally makes the film.