William Lustig and Joe Spinell, say they didn't always have the necessary permits to film on location in New York City. Certain scenes (including the infamous shotgun through the windshield scene) had to have been filmed quickly and afterwards the crew had to run away before the cops arrived.
Gene Siskel was so disgusted by the infamous "shotgun head explosion" scene that he walked out of the movie, saying on his television show with Roger Ebert that the film could not redeem itself after the ultra-violence that he had seen.
The dummy used for the exploding head scene had been used extensively by Tom Savini for effects in Dawn of the Dead (1978). After its use in this film, it was so saturated in fake blood and gore that it was decided to retire the dummy (which Tom had named "Boris"). According to Savini, the dummy was locked in the trunk of the car used in the shotgun scene and sunk in the East River.
The original budget of the movie was $48,000 in cash. $6,000 of which came from Joe Spinell which was part of his $10,000 salary from the movie Cruising (1980) that he recently completed before filming began. $12,000 came from Andrew W. Garroni and the rest ($30,000) came from William Lustig which was from their profits in the adult film business. The three of them put all that money into a stock market account and the amount grew to $135,000 as production continued. It was British producer Judd Hamilton who came up with the rest of the money (around $200,000) to complete the movie as part of a condition that his then-wife, Caroline Munro, would be cast as the heroine.
Joe Spinell was working in between the filming of Maniac (1980) with other movie projects, one of them being Nighthawks (1981) which began filming before production on Maniac ended, in which Spinell cut his hair short and shaved off his mustache to play a clean-cut, high-ranking New York police official for Nighthawks. In a few scenes (most of them being where Frank Zito is driving his car), Spinell is wearing a fake mustache with a long-haired wig under his cap.
The film's distributor had a unique campaign to support its release in New York: mini-screen kiosks were set up in front of theatres that were showing the film in 1980 that had several minutes of uncut footage, including gory murders. However, the campaign backfired when Gene Siskel condemned it on both his regular TV reporting and "At The Movies", leading to a backlash against the film's violence and the cancellation of plans to have the kiosks used in such major markets as Chicago and Los Angeles.
The scenes in Frank Zito's tiny apartment were inspired by the Swedish thriller Man on the Roof (1976) with the claustrophobic setting and the quiet and suttle tone with the sounds of a dripping faucet, a ticking clock, and occasional sound of traffic from outside. The color and crude decor of the apartment and other sets were inspired from the color-theme sets of Italian horror thrillers such as Deep Red (1975), Suspiria (1977), Blood and Black Lace (1964) and several others.
The 1970s dark blue Buick Electra car that is driven by the killer was owned at the time by director William Lustig which was purchased from a used car lot for $200. The car was permanently damaged late in filming when a camera mount set up by the crew to film a scene with Frank and Anna in the car inadvertently damaged the steering wheel mechanism wire which forced Lustig to get rid of the car which became impossible to steer afterwords.
According to director William Lustig, Italian horror director Dario Argento was supposed to be involved as co-producer because his wife at the time was originally offered the protagonist role but was unavailable. Also Lustig's original request for the music was to have Goblin, Argento's Italian rock metal band to the score the movie Maniac before using Jay Chattaway as a second choice.
The opening shots of Rita entering her apartment and all of the subsequent scenes inside her apartment were the very first scenes to be filmed. The climatic murder and mutilation of Frank Zito by the zombie-mannequins was the last scene to be filmed.
The montage of Frank Zito looking at department store mannequins late at night was filmed by Joe Spinell's friend and assistant Luke Walter as two-man second unit after the rest of the crew were asleep for the night.
The movie was never given a rating by the MPAA. The filmmakers knew it would receive the dreaded X rating for the violence and gore contained and simply refused to submit it for any review, because at the time it was easier for unrated films to be shown in theatres than X-rated ones due to the latter always being seen (even if, as in this case, inaccurately) as pornographic.
Michael Sembello recorded a version of "Maniac" as the title track for a soundtrack for this film that was never released. While the lyrics are lurid and violent as reflections of what the movie was like, the song later caught the attention of producers who got Sembello to record a new version that matched the film they were working on. That film was "Flashdance" and "Maniac" became a #1 hit song on its soundtrack.
A 1979 post-production ad for Variety magazine stated some of the cast as including Daria Nicolodi, Susan Tyrrell and Jason Miller opposite Joe Spinell. Nicolodi was offered the lead role but was unavailable, and no information has yet surfaced to reveal which roles were to be played by Tyrrell and Miller.
Director William Lustig stated that co-writer and star Joe Spinell had to prepare for the lead villain role of New York serial killer Frank Zito by doing research on real life serial killers such as Ted Bundy, John Wayne Gacy, David 'Son of Sam' Berkowitz, and others as well as he went to strange excite places to really get into the role. Also, Lustig stated that there was even a real murder of a prostitute in the same hotel (the Hotel St. James) where they were filming Joe as Frank's scene killing a prostitute around the same time and the real life killer was never caught. People dressed up fake mannequins with the victims clothes to see if they can identify them as well what the character Frank does in the movie with his victims.
When the film's poster appeared on a billboard on the property of the R&B Custom Shop in West Hollywood, the owner was so disgusted by the violence displayed she and several of her friends actually painted the billboard white.
Louis Jawitz, who played the role of Anna's art director, was actually a professional photographer who leased his studio to be used for one day to film the scene with Frank visiting Anna and the models. Aside from his fee, Jarwitz only other request was to appear in the film as Anna's art director in a cameo appearance.
The trivia items below may give away important plot points.
Because they would only have one chance to film the scene where Tom Savini's character gets shot, Savini decided that he should be the one to pull the trigger. He said it felt a little weird shooting the dummy he had created of himself in the face.