James Whale's 1931 classic film, Frankenstein spawned six direct sequels. In The Bride of Frankenstein, The Monster learned to speak. In the next film, The Son of Frankenstein, the screenwriters dropped the idea of a talking Monster, and invented the character of Ygor, the doctor's assistant. In The Ghost of Frankenstein Ygor's brain is transplanted into The Monster, and after the operation The Monster speaks … with Ygor's voice. And then, because brain transplants can be tricky, The Monster went blind.
And now a little backstory …
After the success of Dracula, Bela Lugosi was offered the role of The Monster in Frankenstein. Lugosi considered the part to be beneath his talents, said he was a star in his own country, and did not come to America "to be a scarecrow." William Henry Pratt, a struggling British actor, took the part, changed his name to Boris Karloff, and became a movie star.
Karloff played The Monster in the first three Frankenstein films. Bela Lugosi played Ygor in The Son of Frankenstein and in The Ghost of Frankenstein. Lon Chaney Jr. assumed The Monster's role in The Ghost of Frankenstein.
At the end of The Ghost of Frankenstein, the laboratory is in flames, Lon Chaney Jr. is stumbling around as the blind Monster speaking in Ygor's (Bela Lugosi's) voice, and finally the roof caves in trapping The Monster in a white hot inferno.
The Monster Movie was the cash cow that kept money rolling in to Universal Studios throughout the Great Depression and beyond. But by 1942, Frankenstein's Monster wasn't the impressive draw it once was. It was time to rejuvenate the franchise by adding another monster into the mix. It was time for Frankenstein Meets the Wolf Man. But there was a problem.