Bela Lugosi was slated for the role of Dracula in "House of Frankenstein" (1944) , but the film was dependent upon the presence of Boris Karloff being released from the stage tour of Arsenic and Old Lace. Shooting was delayed, and John Carradine was cast instead of Lugosi, who had a prior engagement: ironically, playing Karloff's "Jonathan Brewster" role in another touring company of "Arsenic and Old Lace."
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Glenn Strange was the fourth actor to play the Monster in Universal's Frankenstein series. The actor who played the original Monster, Boris Karloff, was of course on the set, playing the role of Dr. Niemann so he was able to personally coach Strange in the way the Monster should be played.
The only actor to reprise his original monster role was Lon Chaney Jr. who played Larry Talbot, aka "The Wolfman", for the third time.
By the way Kharis the mummy, another Universal "classic monster", was to be in the movie but was removed because of budget restrictions. Lord knows who they would have cast in that role? Possibly Lugosi?