These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
In real life, Vincent Price was an excellent chef and art expert. In the movie, Phibes cooks up a Brussels sprouts-based concoction in order to get locusts to eat a woman, and throws a withering Death Glare at one of his victims upon seeing an ugly piece of art in his room.
The wax busts he melts after each kill. And, indeed, the casting of Price as a vengeful and disfigured burn victim.
Dr. Phibes' wife's full name is Victoria Regina Phibes. Victoria Regina was the name of a 1935 play starring Vincent Price.
Older Than They Think: The last challenge, based on the Plague of the Death of the First-Born Son, requires the surgeon who failed to save Phibes' wife to cut out a key planted from behind his own son's heart that will allow him to deactivate a machine that will spray acid into his son's face if he takes too long getting the aforementioned key. This might as well be a prototype for the Saw films.
Also, this is a film about a man with burned skin, supposedly dead, who takes revenge on those responsible, sometimes through their children. Hmmm.
And although the idea of the Poetic Serial Killer goes back way before this movie, it's fair to say that Se7en may have gotten some inspiration from Phibes' extreme creativity and use of religious motifs.
Squick: "And then he'll have a face... 'LIKE MINE!'"
What Could Have Been: Peter Cushing as Dr. Vesalius. He turned the role down because of the serious illness of his wife. He got a cameo in the sequel though. And, you know, given that they ended up with Joseph Cotten, it's not a total loss.
The sequel, Dr. Phibes Rises Again, contains YMMV examples of:
The Other Darrin: Vulnavia. Virginia North was pregnant and therefore unable to reprise the role. Valli Kemp was cast instead.
Reality Subtext: This was the last film Price did for American International Pictures, who were planning on replacing him with Robert Quarry as their main horror star (it didn't go too well. You've barely ever heard of Quarry, right?). Some of the dislike between their characters is genuine.
Even more uncomfortable in Madhouse, where Price plays an iconic horror actor, and Quarry plays an exec who is deliberately trying to ruin his career.
What Could Have Been: Various ideas for a third film were discussed, including protecting the eternal life secret from Nazis, Phibes fighting a group of Satanists led by the grandson of Vesalius, Phibes fighting another Satanist (Christopher Lee and Donald Pleasance were considered for the part, and there would have been a scene where Phibes puts butter on one guy and boils him like a lobster) in Greece and eventually ascending to a higher plane of existence. Victoria would be an active character in the third movie, and serve as her husband's Morality Pet. At least one of the scripts was to have given the Clockwork Wizards mechanical band the power of full movement, and have them engage in a street battle with a squad of armed policemen. Several scripts were written (including one titled Brides Of Phibes, which was planned to co-star Roddy Mc Dowall as Phibes' adversary), and a few of these met Price's approval, but his ailing health prevented their production.