Acting for Two: In addition to playing Irena, Simone Simon dubbed the voice of Elizabeth Russell when she calls Irena "my sister" in Serbian.
Actor-Inspired Element: Val Lewton overheard Simone Simon singing a specific tune, so he took her to composer Roy Webb - who then developed the tune into the film's score.
A positive example. Supervisor Lou L Ostrow tried to get Val Lewton fired after being dissatisfied with the rushes after three days of shooting. He also tried to have Jacques Torneur replaced as director. RKO executive Charles Koerner overruled him, as he was satisfied with the work done already.
For the drafting room scene nearer the end of the film, Jacques Torneur wanted to only suggest the presence of an animal via shadows. The studio however insisted on an actual panther being used in the scene - so he compromised by shooting it ambiguous enough that "you couldn't be sure what you were seeing."
Fake Nationality: Irena is Serbian, and played by French actress Simone Simon.
Follow the Leader: Several other horror films eventually followed, all revolving around a female character who fears she may turn into a monster. These included Jungle Woman, The Woman Who Came Back, She-Wolf of London, The Catman of Paris, The Cat Creeps and The Soul of a Monster.
Hostility on the Set: Simone Simon proved to be extremely temperamental, behaving quite horribly to her co-star Jane Randolph. The latter claimed she often "upstaged" her in their scenes together and at one point deliberately poured coffee on her costume to delay production. Jacques Torneur confronted Simone over her bullying, and they ended up having a shouting match in French for everyone to hear.
No Budget: Only made for $150,000 - which worked in the film's favour, forcing the filmmakers to shoot in shadows to increase suspense.
Sleeper Hit: This B-movie was a Box Office success and was in theatres for so long that many critics went back to see it a second time. A lot of the ones who had written negative reviews changed their minds on the second watch. It was actually such a hit that Val Newton's next two films had their releases pushed back.
Uncredited Role: Elizabeth Russell is uncredited for her one scene as the Serbian woman who speaks to Irena at her wedding party.
The first draft of the script leaned closer to a short story "Ancient Sorceries" by Algernon Blackwood - taking place in a French village with lots of medieval architecture and inhabited by devil-worshiping cat people. Just as the rights to the short story were about to be bought, Val Lewton changed his mind and said it would be set in New York and involve a Love Triangle.
Val Lewton wanted to cast Jennifer Jones as Alice. But Executive Meddling prevented her from doing the film, and so the part went to newcomer Jane Randolph.
Write What You Know: The sequence of Alice nearly being attacked in the swimming pool was based off an experience Jacques Torneur had when a friend's pet cheetah escaped while he was using the pool.
Romance on the Set: Paul Schrader and Nastassja Kinski had an affair on-set during filming. After she called it off, he confronted her in Paris and she bluntly explained that she fucked all her directors.
