Twentieth Century Fox co-financed the film because it's predecessor Suspiria (1977) had been quite a successful film for their company. However, Fox was not the only financier. Producer Claudio Argento acquired additional financing from German and Italian consortiums.
In an interview with assistant director Lamberto Bava, he said that he handled and wrangled so many cats during the shooting of this film that afterward he could no longer stand to be in the same room as a cat. He's avoided them since then.
Dario Argento once said that the gentleman who provided the live ants used in the film collected them by walking around in the park with a vacuum and literally sucking them up from the ground. He would later retrieve them from the vacuum bag once on set.
According to co-writer and star Daria Nicolodi she didn't fight for writing credits on this film as she had an ordeal just getting writing credit on Dario Argento's previous film, Suspiria (1977). According to Nicolodi the basic plot of 'Inferno' was her creation. Reportedly, Argento wrote the screenplay working from Nicolodi's original story notes whilst staying in a room in a New York hotel situated with an aerial view of the city's Grand Central Park.
When star Leigh McCloskey's stunt double broke his leg, McCloskey himself had to perform the stunt work for the films explosive finale. In interviews McCloskey said it was an intense experience as the rest of the crew and equipment were protected by multiple layers of Plexiglas while he had to run without protection through sets rigged to explode and burn. McCloskey said 'when you feel glass flying by you like a Harrier jet, you never forget it!.
Reportedly Dario Argento was ill with a serve case of hepatitis through out the production. At one point he had to be bed-ridden for a few days leaving the production to work on only second unit. Argento has since called 'Inferno' perhaps his most challenging film for this reason alone.
For the scene where Kazanian carries the bag of 'cats' into Central Park a mechanical device was placed inside the bag to make it move, giving the impression that there were actually live animals inside.
Mario Bava performed a number of uncredited roles on this film. Bava was uncredited for the following duties: camera operator, lighting technician, visual effects artist, second unit director and even acting as a full director directing performances.
Director Dario Argento has frequently cited this picture as being one of his least favorite of his films due to the production difficulties associated with it due to the pain of the meningitis illness he suffered whilst making it.
According to the book "Mario Bava: All the Colors of the Dark" (2007) by Tim Lucas, Sacha Pitoëff's death scene was filmed on location in Central Park during the summer of 1979. Production Coordinator William Lustig said of this: "They filmed the actor carrying a bag that contained some kind of moving mechanism, to make it look like it was full of cats. He walked into the lake, pushed the bag underwater, and fell in. At that point, some phony mechanical rats were attached to him for closeups. When the guy at the hamburger stand runs over the lake... that guy was actually running on a Plexiglas bridge under the water; it made it look like he was actually running across the surface of the lake. All of the stuff with the live rats was shot back in Europe".
The trivia item below may give away important plot points.
In Kazanian's bizarre murder scene, it appears that the knife-wielding hot dog vendor runs across the pond to him. A Plexiglas bridge was placed just under the surface of the pond to make it appear that the vendor runs across the water guided by supernatural forces.