Actor Ronny Cox, who plays Eli MacCleary, also wrote and performed the country music featured in the film.

Paul Clemens kept the jacket that he wears throughout the film. He eventually loaned it to an actress he was dating who wanted to use it for a photo shoot, and she made the mistake of having it dry cleaned. The fake leather sleeves melted and the jacket was destroyed.

The film's writing credits are odd and misleading. Author Edward Levy sold off the screen rights to the title "The Beast Within," but he found himself with a case of writer's block and missed the publisher's deadline for his book. The producers presented the title to screenwriter Tom Holland, who created an original story for his screenplay. With the movie pushing into production, Levy quickly composed a novelization of Holland's script, though Holland received no story credit for the book.

Writer Tom Holland felt that the transformation scene that the public was sold on was not handled properly somewhat. He felt that Director Philippe Mora lingered "too much" on Paul Clemens and not cutting away to see the reaction shots by the other actors in the room which would've made the effect alot more shocking than it really was.

Paul Clemens (Michael) has described the scene in the kitchen with Logan Ramsey (Edwin) as the most difficult scene to film in his entire career. Ramsey kept going off script and Clemens found it practically impossible not to laugh at his various ad-libs.

The film was a lot gorier than the final cut that was shown theatrically due to the MPAA.

The names "Curwin" and "Dexter Ward" are characters from the horror novel "The Case of Charles Dexter Ward" by Howard Phillips "H.P." Lovecraft.

Along with Joe Dante's The Howling (1981) this film pioneered the trend of air-bladder special effects makeup. For Michael's transformation scene small plastic sacks (often condoms or balloons) would be embedded into the layers of makeup and face castings. Later while filming these sacks would be inflated through tubes and it would help to give the appearance of the skins distortion. The same technique became popular in the genre throughout the 1980s being used in such horror films as Demons (1985), Fright Night (1985), and Evil Dead II (1987).

Michael's x-rays were actually x-rays of actor Paul Clemens. Strips of metal were applied to Clemens to create the illusion of subcutaneous anomalies.

This was the feature film debut for horror genre regular Tom Holland - who wrote the script.

Paul Clemens is double-jointed so he was able to contort his hands in ways that appeared unnatural.

The film was released towards the very end of the horror transformation period of horror films that finally culminated with John Carpenter's The Thing. The other films included An American Werewolf In London and The Howling which caused the creation of a special Oscar Category for Visual Effects.

This was the final feature-length score for composer Les Baxter, who considered it to be one of his finest.

The interior and exterior of the power plant were actually the same location. These scenes had to be dubbed because there were roaring turbine engines running which were so deafening that the cast/crew had to stuff their ears with cotton.

Writer and eventual Director Tom Holland, was still acting during the time he had written the script for this film and eventually Psycho 2 a few years later after he had given up acting to write full time.

The movie was filmed in 1980. The attempted assassination of President Ronald Reagan by John Hinckley occurred during the filming of the big transformation sequence, which was shot over the course of four days.

One shoot, at an abandoned hospital, fell on Friday the 13th. The crew became convinced the location was haunted as throughout the evening the lights and the elevator turned on and off by themselves.

Writer Tom Holland had written the "transformation" scene for Michael to completely shed his skin just like a cicada. "An insect like cicada head as he sheds his skin. His face eventually falls off and split in half" as the script originally intended. Holland considers it "a little laughable now" due to the air bag effects used in the sequence.

The films French title is Les Entrailles de l'enfer, which translates to "The Entrails of Hell" in English.

Director Philippe Mora once referred to this film as an "encyclopedia" of horror movies.

Writer Tom Holland joking stated that the Michael cicada monster looked like an "alien" than a cicada creature.

Star Paul Clemens was very enthusiastic about having the role of Michael MacCleary because he was an avid fan of the horror genre. Clemens would even enjoy the extensive makeup work that would take hours to apply to him.

This film became a staple on Joe Bob Briggs' Monstervision series, though the network would heavily edit the film.

When Michael is lurking and stalking the members of the Curwin family one by one, a buzzing noise emphasizing the cicada is heard in the soundtrack.

The film is a hybrid of a possession and monster movie according to Tom Holland.

Tom Holland wrote the script which was completely original with the insistence of Producer Harvey Bernhard who had bought the rights for the novel by Edward I. Levy. Levy was preoccupied with a pending divorce that actually prevented him from actually writing his novel and Bernhard's solution was for Holland to write the script featuring a transformation scene turning into the beast.

The translation into English of the German title is "The Angel Face: Three Nights of Horror."

This movie had a warning attached that theaters would have vomit bags at all seats.

During the final rape scene, under the monster suit Paul Clemens was unable to hear Philippe Mora's direction, so Mora put his hand on Clemens' butt to instruct him to thrust. Studio executives were completely baffled when they received the dailies and noticed a mysterious hand fondling the monster.

For the beheading of Judge Curwin, director Philippe Mora shot a cutaway of the judge's bow-tie flipping into the toilet and the bowl being spurted with blood, but the MPAA found this footage more appalling than the decapitation itself, so the shot was removed.

The backstory of Billy Conners transformation into the cicada and his thirst for revenge against the Curwin family. It was that Lionel Curwin, the family patriarch finds Conners and Curwin's wife in a tryst and catches them trying to runaway together. Curwin then chains up Conners in the basement of his house starving him to death while killing his wife in the process. He throws the body of his dead wife in the basement dungeon and trying to survive has to literally eat the woman he loved. This causes him to go crazy and using the Indian curse, "Song of Ciccada" which Tom Laws taught him which allowed him to survive the ordeal and pass on the curse to his progeny if he's able to impregnate a woman and get his revenge against the Curwin family. According to Writer Tom Holland, all of this was completely cut out of the final cut of the film and leaves a major plot hole that is only vaguely referenced during the jail scene with Judge Curwin.

Star Paul Clemens donned the creature suit and performed all of the scenes at the end of the film himself.

When the creature attacks his father (Ronny Cox) at the end of the movie, at one point during the fight it twists Cox's arm around his neck, in exactly the same way Cox's character in Deliverance was found in the river.

According to Tom Holland, he used the "Song of the Ciccada" as his plot device and explanation for why Michael turns into the cicada monster since Billy Conners, had used this as a curse towards the Curwin family for revenge. The Song is more of an Indian curse used for him to survive eventually turning into a human cicada much like the creatures themselves.

Tom Holland wrote a complete 84 page spec script that was completely detailed and featured a lot of the background involving the Curwin family, and what happened to Billy Conners, whose remains are only seen in the dungeon in the film, as well as how Conners transformed into a cicada to regenerate himself and rape Bibi Besch's character 17 years earlier.