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?', to which Price replied; '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. Amongst the titles mentioned for the film include; "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 film, 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 written, all exploring different ideas. There was also a plan to bring Dr. Phibes to the small screen, with a TV series, in which a reformed Phibes fought crime. In the 80s, an updated Phibes film was proposed as well. Unfortunately, Price's departure from AIP, and AIP's subsequent change of focus to films which would be of the exploitation/grindhouse fare, meant the third film (nor any further ones beyond it) were 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.
In addition to Vincent Price, five other cast members from the prior film encore, three in the same roles (Peter Jeffrey, John Cater, Caroline Munro) and two in different roles (Hugh Griffith, Terry-Thomas). Four of the six clockwork musicians are also back, but as no actors are credited in either film, it's impossible to know if any of them were returnees.