When Mel Brooks was preparing for this film, he discovered that Ken Strickfaden, who'd made the elaborate electrical machinery for the lab sequences in Frankenstein (1931) and its sequels, was still alive and living in the Los Angeles area. Brooks visited Strickfaden and found that he had stored all the equipment in his garage. Brooks made a deal to rent the equipment, and gave Strickfaden the screen credit he didn't receive for the original films.
When Gene Wilder leans in to kiss Madeline Kahn goodnight in her bedroom, her last-second quip "No tongues" was ad-libbed by Kahn.
Gene Hackman ad-libbed The Blindman's parting line "I was gonna make espresso." The scene immediately fades to black because the crew erupted into fits of laughter. Hackman was unable to repeat the line without laughing with the rest of the crew, so the first take was used. Hackman was uncredited when the movie was originally released in theaters.
The shifting hump on Igor's back was an ad-libbed gag. Marty Feldman had been surreptitiously shifting the hump back and forth for several days when cast members finally noticed. It was then added to the script.
The cast, especially Mel Brooks, had so much fun and were so upset when principal photography was almost completed, that Brooks added scenes to continue shooting.
Mel Brooks: The voice of the original Dr. Frankenstein when Frederick sees the laboratory for the first time.