Actor Glenn Strange suffered greatly during the shooting of the scene in which the Frankenstein Monster is discovered in quicksand. After sitting for three hours in the makeup chair each morning, having his makeup applied by Jack P. Pierce, Strange would spend the rest of the day buried in cold liquid mud (which doubled for the quicksand). "Then everybody else went out for lunch," Strange recalled. "By the time they came back, I was so cold, I could barely feel my legs." Strange's co-star, Lon Chaney Jr., suggested that Strange use alcohol to keep himself warm. Throughout the day, Chaney passed a bottle of whiskey to Strange in between takes. By the end of the day, Strange recalled, he was so drunk he could barely dress himself after removing his monster makeup and costume.
Although the Frankenstein Monster is found still clutching the skeleton of Dr. Niemann after wandering into quicksand in House of Frankenstein (1944), the resurrections of Larry Talbot/the Wolf Man and Count Dracula from their "deaths" in the same film are not explained.
The trivia items below may give away important plot points.
According to the Universal Film Script series entry for "House of Dracula", the film grew out of an earlier script, "The Wolf Man vs. Dracula", a proposed follow-up to Frankenstein Meets the Wolf Man (1943) in which Talbot (Lon Chaney Jr.) would do battle with Dracula (Bela Lugosi, to be doubled by a "giant bat". At the climax, villagers attack the house and the Wolf Man kills a large number of them. The Hays Office flat-out rejected the script as too violent, so a more toned-down version was written, and eventually became this rather tame film (although Lionel Atwill does get electrocuted again, this time by Dr. Edelmann.)
The climatic scenes of the Monster trapped by the fire were "borrowed" from the ending of The Ghost of Frankenstein (1942), where Lon Chaney Jr. played the Monster. Therefore, when Chaney (as Talbot) shouts to the villagers to "Get out! The Frankenstein Monster!", he's actually running away from himself (and Eddie Parker, who doubled for Chaney in the earlier film).