It was reported that the rubber headpiece used for the Frankenstein monster make-up was very uncomfortable for Lon Chaney Jr. to wear. It sat directly on his forehead and he constantly complained. Once he asked for it to be removed. Angry and frustrated when no one listened, he ripped it off himself, tearing open a bloody gash in his forehead. Production on the film was shut down for a couple of days.
Lon Chaney was known for his hard-drinking ways. During production of this film, Chaney became inebriated while in full costume and got "lost" in the intricate mazes that were part of the laboratory sets. It took several minutes for him to find his way free. A similar incident occurred many years later when Chaney played the Frankenstein Monster on an episode of Tales of Tomorrow. Once again, Chaney became intoxicated and mistakenly believed that the show's live telecast was actually the final dress rehearsal. For much of the performance, Chaney stumbled about, picking up breakaway props he was supposed to destroy and then setting them back down.
Ygor and the Monster weren't the only characters who came back from the dead. Michael Mark and Lionel Belmore, who play the two council members murdered in Son of Frankenstein (1939) are back as council members in this one, seemingly none the worse for wear.
Dwight Frye appears as 2 different characters. First as one of the villagers who destroys the Frankenstein castle at the beginning of the film. Later in flashbacks to the original Frankenstein (1931) where he played Fritz, the hunchbacked assistant to the original Dr. Frankenstein.
The first draft of the script was written by Eric Taylor was considered too depressing. The original Taylor treatment brought Wolf von Frankenstein back into play, as well as Ygor, plus a misshapen hunchback, Theodor. Ygor's plan was to create a vengeful mob of society's rejects, ala Freaks (1932), led by himself, with the Monster as brute force. A rewrite was ordered and given to veteran writer Scott Darling who retained the fundamental scenario but made significant changes.
Boris Karloff, then acting in the hit Broadway show which became Arsenic and Old Lace (1944), had no interest in working on the film. Producer George Waggner wisely decided to retain Karloff's make-up out of fear that the public would not accept any change in the monster's appearance.
The trivia item below may give away important plot points.
Lionel Atwill's character (Dr. Bohmer) is responsible for ensuring Ygor's (Bela Lugosi) brain is switched with that of the monster (Lon Chaney Jr.) making him far more dangerous than he was before. In Man Made Monster (1941) Atwill plays a mad doctor who makes Chaney Jr.'s character dangerous via electricity (which originally brought life to the Frankenstein Monster)