According to the book "The Films of Myrna Loy" by Lawrence J. Quirk: "She recalls that she and Karloff decided between themselves that the only intelligent way that this movie could possibly be played was subtly tongue-in-cheek."
In filming the scene where Fu Manchu injects his mind control drug into Terry Granville's neck, Boris Karloff actually pushed the syringe into a baked potato, which was lying on the table next to Charles Starrett's head, out of camera range. However, each time Karloff pressed the plunger down, the potato would explode. This happened on several takes, until Karloff and Starrett couldn't do the scene without laughing. Director Charles Brabin finally gave up and dismissed the two actors for the day, saying, "Never mind! We'll shoot it tomorrow morning!"
As originally scripted, during the torture sequence, it was planned to have Von Berg (Jean Hersholt) suspended over the crocodile pit and Neyland Smith (Lewis Stone) about to be impaled by the walls of spikes. This was reversed when, for reasons unknown, it was thought that a fat captive would make more sense being impaled than a thin one.