Gene Wilder, Peter Boyle, and Marty Feldman appear together in this film by virtue of the fact that their mutual agent had a deal with the movie studio.
The original cut of the film was three times the length of the final cut.
Another time you can see Gene Wilder laughing when he's not supposed to be is after he finds out the project failed (the quiet, dignity, and grace speech). He starts strangling the monster, and you can see that he is stifling laughter. Finally, he can't hold it in any longer and proceeds to partially hide his face behind the monster lying on the table in an attempt to cover up his smirks (but you can still see that he is laughing).
The howling wolf sound on the ride to the castle was made by Mel Brooks.
Mel Brooks is in every one of his movies. Even though in this one you don't see his face, when Gene Wilder hits the cat with the dart, Mel Brooks did the sound of the cat.
The scene where Igor says, 'Walk this way,' meaning to follow him, and Dr. Frankenstein imitates his limp, was the inspiration for Aerosmith's classic song 'Walk This Way'. The band had written out some lyrics and part of a tune, but they needed a continuing line to make it click. When they caught a late night showing of Mel Brooks's masterpiece, and they saw the scene, they knew they had it.
The cast and especially Mel Brooks had so much fun and were so upset when principal photography was almost completed, that Mel added scenes to continue shooting.
The song and dance number with Dr. Frankenstein and the monster almost didn't make it into the film. Director Mel Brooks thought it would be too cheesy and wouldn't do anything to advance the plot. Gene Wilder, (who wrote the script) wanted to show how tenuous the doctor's control over the monster was. So, he argued for the scene. Mel was adamant, Gene was more so. He argued for it until he was blue, then red in the face. Finally, Mel allowed it in the film saying, "It felt wrong but I knew that if you fought hard enough for it, it was right."
The assistant property master's name, Charles Sertin, is on the third brain on the shelf.
Throughout this movie, Gene Wilder barely stifles genuine laughter. An easy spot to find this is when Igor bites the fur of Madeline Kahn's mink. Wilder says "Stop that." and can be easily seen laughing.