Trivia: At first, Lou Costello did not want to be in the film because he thought the movie was too silly. But when Universal (the company that made the film) offered him $50,000 to be in the movie, Costello said he would be delighted.
Trivia: The voice of the Invisible Man was provided by actor Vincent Price.
Answer: He's a rather simple-minded guy. He was scared and had become so overly-immersed in pretending to be hypnotized that it took time for him to act normally. Also, it's a movie. They're going for comic effect.
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