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)
According to contemporary publicity reports, Bela Lugosi was put under hypnosis by technical advisor Manley P. Hall to make his death scene more realistic and harrowing.
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.
Long after Bela Lugosi's death, it was revealed that the "hypnosis" angle was a complete hoax, but Boris Karloff played along at the time of the film's release, stating that he was convinced that Lugosi had been hypnotized, because he had never seen his fellow co-star keep his back to the camera so long.
Originally budgeted at $130,750.
Part of the SON OF SHOCK package of 21 titles released to television in 1958, which followed the original SHOCK THEATER release of 52 features one year earlier.
Shooting lasted from December 28, 1939-January 18, 1940, released March 21.
The only one of the eight films featuring Boris Karloff and Bela Lugosi in which the two stars have no scenes together.