Sandy Alexander who plays the prisoner who harasses Robert Forster in the film was in fact was the leader of the New York Hell's Angels unbeknown st to director William Lustig and after the film was released, Alexander was convicted of murder.
Joe Spinell, who played the title character in William Lustig's Maniac, was cast in the small role of the corrupt lawyer Eisenberg. During the filming of the courtroom scene, Lustig recalled that Spinell's escalating alcohol and drug abuse was becoming a problem as Spinell would often show up late for filming and often drunk or stoned or both which slowed production. One morning, Spinell did not show up at all which led Lustig to shut down filming for half the day while his producer partner, Andrew Garroni, had to go looking for Spinell on the streets from various bars to hotels where he might be. The scene where Eisenberg arrives late in the courtroom after speaking with Prago was filmed in late afternoon on that day that Spinell did not show up that morning for filming.
The scene where Willie Colon is killed by Robert Forster, the actress that played Colon's girlfriend was jerked so hard backwards that the cast and crew worried for her safety after landing hard in the bathtub. This was a complete accident and she was a "trooper" according to William Lustig
The scene after Frank Pesce is being beat up in the bathroom by Richard Bright and Joe Carberry, Fred Williamson picks him up and puts him out the window was done with no safety nets or wires with Williamson holding onto him really tight.
The fight scene in the shower where Woody Strode saves Robert Forster in the shower, Strode actually punched Sandy Alexander who hit his head against the wall and ended up dazed after the scene was over.
The film was very popular overseas such as Scandinavia, Germany and in particular, England where it was the number one video rental for six months. The only place according to director William Lustig that the film "didn't do anything" was in Japan.
After the gang leader takes the 60 foot fall after Robert Forster chases him and dies, William Lustig felt that the blood splatter wasn't juicy enough and re-shot the scene after going over to the catering table and using a tub of cream cheese mixing it with fake blood then pouring around the actor for the look Lustig was aiming for according to the DVD commentary.
According to William Lustig, the nighttime scene where the Vigilantes follow Horace the pimp was shot "In one of the worst neighborhoods in (Greenpoint), Brooklyn. Everything was spooky and burned out" on a crane camera with no real safety measures and regrets not using the "poor man's" process for the scene which would've been much safer."
In the scene where Fred Williamson is chasing Frank Pesce, Richard Bright hits him with what appears to be a real baseball bat. In fact, it's a plastic bat in which Bright actually hit Pesce on the ear and his face. His facial expressions were real. Pesce joking states on the DVD "He's swinging like an idiot...that fucking stung. He hit me thinking it was a real baseball bat. Lee Strasberg stuff".
Train sound effects were added in the scene where Fred Williamson is interrogating Horace the pimp because the scene was actually shot in the parking lot of the swimming pool area and according to William Lustig "wanted the scene to have the feeling of being under an elevated train"
The second film in which Rutanya Alda has had a child shot by a shotgun. In this film, her and Robert Forster's son was killed by the gang and in her previous film, Amityville 2, her children (and herself) were killed by her possessed son Jack Magner.
The pier scene where the henchman is shot, the stuntman did the stunt so fast that he landed so hard in the water and hurt himself. They had a portable shower on standby to help him get cleaned up according to director William Lustig.
The scene where the two patrol cops were killed by the gang underneath the underpass had to be re-shot according to director William Lustig because when the gang starts shooting at them in the car, their blood squibs didn't go off. They had to completely reconstruct the car the next day to re-shoot the scene and this time the squibs went off as planned.
The hospital where Rutanya Alda is taken to after her brutal attack by the gang was in fact an abandoned hospital in the Bronx. The crew had to bring everything needed to complete the scene and make it look realistic as possible.
The scene where Robert Forster is packing and leaving his house with Fred Williamson confronting him was the very last scene of the movie to be filmed which was shot on the day before Thanksgiving in 1982. It was the coldest day on the shoot and the cast and crew were working as fast as they could because they wanted to wrap the day's shooting as quickly as possible because "they wanted to go home and see their families or they were going to wrap for him" according to William Lustig on the DVD commentary.
According to William Lustig, the final chase scene with Robert Forster was shot by Randy Jurgensen, who was a NYPD detective and also technical advisor on the film which was done without shooting permits. Jurgensen actually doubled for the gang leader and drove the Datsun for the majority the scene.
The film was cut by the MPAA for violence which included the blood splatter shotgun scene in the pier which was the version of the film seen on theatrically and on video until the uncut version was released in 2007.
According to William Lustig, he was contractually bound to have five songs which were performed by Willie Colon in the film. The hardest part for him was finding places to spot the music in and only used four of the songs. The subsequent soundtrack release featured all the songs Colon wrote because he was a very popular Latin singer and performer.