There has been controversy for decades over whether this film should be considered part of the official Universal Horror series (thus making it a sequel to House of Dracula (1945)) or a non-canon, stand-alone film.
The scene in which Wilbur (Lou Costello) is unknowingly sitting on the lap of Frankenstein's monster (Glenn Strange) required multiple takes. The scene allowed Costello to improvise wildly, which caused Strange to constantly break up laughing during the takes.
Lou Costello did not want to make the movie, declaring, "No way I'll do that crap. My little girl could write something better than this." A $50,000 advance in salary and the signing of director Charles Barton, the team's good friend and the man whom some call their best director, convinced him otherwise.
Bobby Barber was employed for the film as a "court jester". It was his job to keep the energy level up through a series of practical jokes and deliberately blown takes. Often when Lou Costello expected Lon Chaney Jr. to come through the door, Barber would run in wearing a hat and cape and immediately run back out. Bela Lugosi enjoyed Barber's antics as long as he was not the victim. On one particular occasion while filming a scene in which the solemn and sinister Dracula descends a staircase, he was followed by Barber, who imitated his every move. After the cast and crew burst into laughter Lugosi glared at Barber and yelled in his thick Hungarian accent, "We should not be playing while we are working!" and then stormed off the set.
Ian Keith was originally considered for the role of Count Dracula, a part he was up for in Dracula (1931), because Universal originally wasn't interested in hiring Bela Lugosi. According to the audio commentary by film historian Gregory W. Mank on the dvd, Lugosi's manager met with the head of Universal and shamed him into giving Lugosi the role by saying, "He IS Dracula! You owe this role to Lugosi!"
Mr. Peabody and the Mermaid (1948) was shooting near this movie on the lot. Tourists were shocked to see Glenn Strange's Frankenstein Monster having lunch with Ann Blyth in her fishtail costume. Both Strange and Lon Chaney Jr. in his Wolfman make-up were invited to the "Mr. Peabody" wrap party, where they hammed it up in make-up.
According to the audio commentary by film historian Gregory W. Mank on the DVD, this was the second cheapest film made by Universal-International in 1948, and it became the studio's second highest-grossing film of that year.
Although the characters had previously appeared in House of Frankenstein (1944) and House of Dracula (1945), this is the only Universal film where Larry Talbot and Dracula share both a scene and dialog.
Grateful Dead's Jerry Garcia cited this movie as one that initially terrified him when he saw it as a small child. His father had died less than a year previously and Garcia had trouble watching it. Subsequently he became fascinated with the film's three monsters and they were a major inspiration for him to become, in addition to a musician, a filmmaker and artist.
Although nobody in the film ever meets Dr. Victor Frankenstein or a member of his family, it had been established in earlier studio pictures that the monster was named "Frankenstein" after his creator, within the story-line.
When Wilbur (Lou Costello) sits on the lap of the Frankenstein Monster (Glenn Strange) in the basement of Dracula's castle, you can see that, when Wilbur has seen the Monster and he is sliding down his lap, the Monster is almost smiling. This is because Costello was ad-libbing different reactions to meeting the Monster in the basement and Strange would start laughing.
Even though actors Bela Lugosi, Lon Chaney Jr. and Glenn Strange all reprise their roles from earlier films, this particular film is not considered to be part of the continuous storyline from those movies. Therefore, there was no need to explain, for example, why Larry Talbot is still a werewolf after having been cured in House of Dracula (1945).
Glenn Strange speaks for the first time as The Monster. This film marks the first time since The Ghost of Frankenstein (1942) that the character has spoken, though it does not explain how The Monster has regained his voice.
Despite having worked for Universal for many years, makeup artist Jack P. Pierce was not under contract, but merely an hourly employee. With the changeover from Universal to Universal International came a desire to expedite movies and save money. Pierce was let go, and Bud Westmore and Jack Kevan's more cost-effective rubber appliances were used in place of Pierce's more time-consuming designs. The rubber head appliance that Glenn Strange wore to play the Frankenstein monster was so waterproof and fitted him so tightly that, after a few hours under the hot lights, he could shake his head and hear the sweat rattling around inside it.
In House of Dracula (1945), the previous film in the series, Larry Talbot was cured of his lycanthropy. Why and how Talbot once again becomes the Wolf Man is not explained, lending credence to the theory that this is a stand-alone film and not to be considered part of the original series. This is also supported by the fact that the Monster can again speak in this film.
Universal contract players Dorothy Hart and Ella Raines were originally cast as "Joan Raymond". Hart was dropped in favor of Raines, who asked to be released because she felt that her part would be overshadowed by the monsters. The part was then given to Jane Randolph.
The trivia items below may give away important plot points.
Glenn Strange was playing the Frankenstein monster, but during shooting one day he tripped over a camera cable and broke his ankle. Lon Chaney Jr. (playing the Wolf Man) wasn't working that day, so he put on the Frankenstein makeup/outfit and filled in for Strange, in the scene where Dr. Mornay gets thrown through the window. So Chaney wound up playing two monsters in this movie.
During the final chase scene, when Wilbur and Chick are standing in front of a door and the Frankenstein monster punches through it, Lou Costello deliberately went off his mark and got hit on the jaw. Director Charles Barton liked his reaction, so he decided to keep it in the film.
During the chase in the woods, the Wolf Man attacks McDougal, who is later found alive and injured. Presumably, this means that McDougal is now a werewolf (unless the Wolf Man attacked with his claws, not his teeth).