In spite of Karloff and Lugosi receiving top billing, neither actor would dominate the story-line. Character actor Stanley Ridges took center stage in that regard.
In an interview, Curt Siodmak said of Boris Karloff, who demanded - and obtained - Bela Lugosi's role in Black Friday (1940): "Karloff didn't want to play the dual role in Black Friday. He was afraid of it. There was too much acting in it. It was too intricate." Karloff thus ended up with the part written specifically for Lugosi, inexcusably leaving the Hungarian actor to play a then poorly cast, minor role as an American gangster, instead of being given Karloff's dual role which instead went to Stanley Ridges. In an interesting twist of events, the following year, Lugosi would have the chance to prove how great he would have been in the dual part, as he was cast in a split personality role in Monogram's "Invisible Ghost (1941)" This is total bunk, as Karloff throughout his career proved he was more than capable of playing dual roles.
Director Arthur Lubin would go on to direct all 131 episodes of the Mr. Ed TV series.
Bela Lugosi was originally cast as Dr. Sovac and Boris Karloff as Kingsley/Cannon, but Karloff's interpretation of the gangster was unconvincing and he was replaced by Stanley Ridges. Karloff instead got the role as Dr. Sovac and Lugosi got a minor role, although his second billing remained. (Bojarski, "The Films of Bela Lugosi", 1980) This is total bunk, as Karloff throughout his career proved he was more than capable of playing dual roles.
Despite problems with Boris Karloff, who steadfastly insisted that the studio comply with the union-mandated eight-hour day (a rule largely ignored, except when a major star demanded compliance), director Arthur Lubin was able to bring the picture in on schedule and $5,000 under budget.