Several actors in studio records and casting call lists did not appear in the movie. These were (with their character names) George Ford (Whistling cop), Leda Nicova (Patient), and Bud Geary (Mounted policeman).
The horror movie technique of slowly building tension to a jarring shock which turns out to be something completely harmless and benign became known as a "Lewton bus" after a famous scene in this movie created by producer Val Lewton.
Supervisor Lou L. Ostrow was so dissatisfied with the style of the movie he wanted to replace director Jacques Tourneur after four days of filming. Producer Val Lewton got studio head Charles Koerner to reinstate Tourneur, and when Ostrow insisted on the panther appearing in the drafting room sequence, Lewton had Tourneur use low lighting putting the panther in the shadows.
R.K.O. gave Val Lewton only $150,000 to make the film, resulting in "creative" producing. This forced many of the scenes requiring special effects to be done in shadows which many believe increased the suspense of the film. When studio execs insisted that more footage of the panther be included in the movie, Lewton was able to maintain the budget and the suspense of the film by limiting how many scenes the panther could be visibly seen and told the cinematographer to "keep the panther in the shadows." Thus the panther was only visible in the office and zoo cage.
Val Lewton came very close to being fired after only three days of shooting. Lou L. Ostrow, the head of RKO's B-unit, had looked at the first three days of rushes and was not happy with what he saw. Ostrow wanted Lewton out, but was ultimately overruled by RKO chief Charles Koerner who was happy with Lewton's work and wanted him to continue.
Near the end of filming, two units were shooting around the clock to speed completion of the film. During the night, one unit would film the animals for the Central Park sequence, while during the day, the other unit would be working with the actors.