Joseph Cotten would grumble on the 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 script Phibes was abusive to Vulnavia, eventually stabbing her to death, and then escape 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 sequences were removed.
Originally advertised in the U.S. with the tag line, "Love Means Never Having to Say You're Ugly," a parody of a famous line from "Love Story," out the previous year. However, that was replaced after the first week, as opening box office was disappointing and 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.
Writer William Goldstein has said that Vulnavia was originally meant to be another clockwork device, complete with a wind-up key in her neck. Believing that Phibes needed a more mobile assistant, director Robert Fuest made her human, but retained the blank facial expression and mechanical movements.
Virginia North appeared in a play in London called "Council of Love," in which she was silent throughout her performance. "I played the daughter of the devil," she told the Associated Press in 1971. "I'd go around and do terrible things. The people who made the movie saw me in the play and decided I was what they wanted. You might say I was ready made for the movie part."
Afro-Canadian composer Shelton Brooks, who wrote the song "The Darktown Strutters Ball" that plays an important role in the film, was born in raised in Amherstburg, the same Ontario town that star Vincent Price spent his childhood summers at.
Dr. Phibes' murders were inspired by the 10 plagues of Egypt found in the Old Testament: 1. Blood (the blood is drained from Dr. Longstreet's body); 2. Frogs (Dr. Hargreaves's throat is crushed by a mechanical frog mask); 3. Bats (Dr. Dunwoody is mauled to death by bats); 4. Rats (Dr. Kitaj crashes his plane when he is attacked by rats); 5. Diseased animals (Dr. Whitcombe is speared by the horn of a brass unicorn head); 6. Boils (Prof. Thornton is stung to death by bees; it's referenced but not shown); 7. Hail (Dr. Hedgepath is frozen to death by a machine spewing ice); 8. Locusts (Nurse Allen is eaten by locusts); 9. Darkness - This may refer to a) Phibes drains the blood from his own body while injecting embalming fluid, apparently joining his wife in death, or b) the depiction of a solar eclipse at the very end; and 10. Death of the firstborn (Phibes kidnaps and attempts to kill Dr. Vesalius's son). The original Biblical plagues of gnats and flies were replaced with bats and rats due to issues with filming.