At this point in his career, J. Carrol Naish was very ill and frail and could no longer remember dialogue, so he read it off cue cards. However, he had only one real eye, so in his dialogue closeups you can see one eye moving back and forth, reading the lines, while the other eye remains fixed in position.
Much of the electrical lab equipment in Duryea's lab are props originally used in Frankenstein (1931). Ken Strickfaden, who had designed all the electrical gadgetry in that film, supplied the equipment.
Originally planned as a sequel to Satan's Sadists (1969), with Russ Tamblyn and other "bikers" reprising their parts from that film. However, not long after filming began, it was decided to turn it into a horror film instead of a biker picture and much of the footage with Tamblyn and other actors from the first film was cut out. They were unable to cut them completely out of the movie, though, which is why Tamblyn and his biker gang seem to be wandering in and out of the film, with no connection to the story line and with not much to do.
It was originally intended to have Dracula turn Frankenstein's Monster into a bloodthirsty vampire, so the Monster could better serve the Count's purpose. The idea was dropped, however, when the fangs kept falling out of actor John Bloom's mouth, which he couldn't keep in due to his heavy makeup.
In his scene confronting Count Dracula, J. Carrol Naish looks noticeably older than he does elsewhere in the film. This is due to the time that had elapsed between the bulk of his scenes, when it was intended as a different film entirely, and the Dracula/Frankenstein scenes that were grafted on later.
Getting special billing as "The Creature," Shelly Weiss actually plays the Frankenstein Monster in the climactic showdown in the church, ending in its dismemberment. This sequence was the only one shot in New York rather than Hollywood, thus the casting change, suggested by Zandor Vorkov, aka Roger Engel.