Trivia: 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).
Trivia: 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."
Trivia: 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.