There was a lot of hostility between Vincent Price and Robert Quarry, particularly when Price discovered that AIP was planning to replace him with Quarry as their major horror film star. At one point, when Price discovered Quarry singing opera, Quarry said "I'll bet you didn't know I could sing did you?" and Price responded "well I knew you weren't a f---ing actor!"
The second part of a planned trilogy. The third film would have featured Phibes fighting a group of Nazis, or searching for the key to Olympus. The title would have been "Phibes Resurrectus," "The Seven Fates of Dr. Phibes" or "The Brides of Dr. Phibes". Proposed stories included Phibes being the subject of an elaborate revenge plot by Dr. Vesalius' son from the first movie, and Victoria being resurrected from the dead, but becoming a deranged homicidal maniac far worse that her husband. Reportedly a large number of sequel scripts existed, all different. Also pitched was a Dr. Phibes TV series, in which he reformed and fought crime, and in the 80s a contemporary Phibes film was proposed as well. Price's departure from AIP, and AIP's change of focus to exploitation/grindhouse fare, meant the third film (or any further ones beyond it) were never made.
At one stage, American International Pictures planned to revive Count Yorga, from Count Yorga, Vampire (1970), as an adversary for Dr. Phibes. The plan was dropped, but Robert Quarry, who plays Count Yorga, appears in this film.
Peter Cushing was originally cast in the Joseph Cotten role in the original film, but withdrew due to his wife's ailing health. He was able to supply a cameo in this film, a day's work (the film was shot in December 1971).
Scenes involving Beryl Reid and Terry-Thomas were cut from the original print due to arguments between the producers. Vincent Price had to be recalled to record additional dialogue to cover the gaps this created in the plot. This is the reason why Phibes says so many lines while not wired up to his speaker.
If the film feels like things are missing it may be because of two reasons. The script was written by two people who did not collaborate but wrote separate drafts that were then sort of stuck together. Even more reason for a jumpy quality to the film is that ten minutes were cut out of the film by American International.