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: The monster may have seemed obedient, but its overall behavior is unknown to us. It's possible that it was at times defiant, obstinate, or just didn't follow instructions completely or competently. It's also just a plot device for a silly movie. There has to be some reason, however flimsy, why Dracula wants Wilbur's brain.
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