Joseph Cotten would grumble on the film's set that he had to remember and deliver lines, while Vincent Price's were all to be post-dubbed. Price responded "Yes, but I still know them, Joe." In fact, Price was well-known in Hollywood for his ability to memorize all of the characters' lines in a given production, not just his own.
In the film's original script Phibes was abusive to Vulnavia, eventually stabbing her to death and then escaping his house (which was to catch fire) in a hot air balloon with Victoria's body. It was decided to make Phibes a more sympathetic character, so these scenes were cut.
For visual reasons the plagues of flies and gnats were replaced in the film with rats and bats.
Originally advertised in the U.S. with the tagline "Love means never having to say you're ugly," a parody of a famous line from the film Love Story (1970), which had been released the previous year. However, that tagline was replaced after the first week due to disappointing opening box office numbers as it turned out audiences had no idea what sort of film this was supposed to be. A new advertising campaign made it clear it was a horror film, and afterwards it became a box office hit.