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The working title of this film was Buried Alive. Although the onscreen credits list only William Bruckner and Robert F. Metzler as the film's screenwriters, the picture was based on Gaston Leroux's 1912 novel Balaoo. According to the Twentieth Century-Fox Produced Scripts Collection, located at the UCLA Arts-Special Collections Library, Frances Hyland also contributed to the screenplay. According to the file on the film in the MPAA/PCA Collection at the AMPAS Library, the PCA was reluctant to approve an June 11, 1942 version of the script. The PCA told the studio: "The story is based on theories of human origin in such a way that, if presented to the public, will undoubtedly offend the sensibilities of large groups of religious people of different faiths, and, accordingly, could not be approved under the provisions of the Code. Secondly, there seems to be an excessive number of gruesome and detailed killings which could not be approved." After conferring with executive producer Sol M. Wurtzel and studio official Jason S. Joy, a PCA official noted in the film's file: "It was agreed to eliminate any reference to Darwin or to his theory, and to establish the ape as a throwback. Details involving gruesomeness will be handled with care."
The picture marked Harry Lachman's last film as a director. While a studio publicity item announcing that Dr. Renault's Secret was to be the first horror film produced by Twentieth Century-Fox was technically correct, Leroux's novel had previously been filmed by the Fox Film Corp. in 1927 as The Wizard. The silent film was directed by Richard Rosson and starred Edmund Lowe and Leila Hyams (see AFI Catalog of Feature Films, 1921-30; F2.6445). A French version of the novel, entitled Balaoo, was produced in 1913 by Eclair. That version was directed by Victorin Jasset and starred Lucien Bataille and Camille Bardou.