Among the unconventional elements of this film was the soundtrack. At a time (early 1930s) when movie music was usually limited to the titles and credits, Edgar G. Ulmer had an almost continuous background score throughout the entire film.
Director Edgar G. Ulmer, when writing this film, loosely based the villain Hjalmar Poelzig, played by Boris Karloff, on director Fritz Lang. Ulmer knew Lang from the German-Austrian film scene and, though he was a huge admirer of Lang's films, felt Lang to be a sadist as a director.
While working on the film, Edgar G. Ulmer began a love affair with 'Shirley Castle', who would eventually become his wife. At the time, Castle was married to Universal producer Max Alexander. The ensuing scandal led to Ulmer being blackballed from most major Hollywood studios throughout the rest of his career.
The first film collaboration of Bela Lugosi and Boris Karloff, who at the time were unquestionably the two biggest stars of horror film. Despite rumors that the two stars were personally very competitive, this marked the beginning of a pleasant working relationship between the two. While Lugosi and Karloff never became close personal friends, they were quite amicable to each other and enjoyed working together.
The trivia item below may give away important plot points.
Reportedly Universal Studios executives were displeased when they viewed the first cut of the film and so ordered the director to do some extra shooting to tone the violence and horror down. Edgar G. Ulmer did the opposite, and added in the now-famous sequence where Poelzig (Boris Karloff) views his dead but preserved wives before he reveals the fate of Karen to Dr. Werdegast.