A major difference is the involvement of a plot. In the book, there’s almost no plot at all, except that the character is a military man in France. In the movie, the plot is long, explained well, and a little more interesting than the book. The pit and pendulum themselves have very little involvement in the movie, while in the book, they’re almost always involved and the pit is introduced almost right away. Also, in the movie, there are way more characters.
I think Roger Corman did a pretty mediocre job making a movie out of the book. I understand that the movie wasn’t really meant to be based off of the book entirely, but they could have involved the pit and the pendulum a little more throughout the movie. Although the plot was good and had plenty of shock, I did like the plot of the book a little more. If I wrote the script, I would have added a lot more to the plot of the book and kept it. It seems like Corman just made the movie with the title, and only had the pit and the pendulum stay. In short, Corman could have done better.