Because producer Dick Randall was simultaneously making kung-fu films in Rome, a cameo for a Bruce Lee imitator, Bruce Le, was written into this film, even though this scene makes no sense in the context of the rest of the film.
In the only trailer released, one of the final shots shows the chainsaw killer approaching the camera. As this happens, the same scream from Janet Leigh, the scene in Alfred Hitchcock's Psycho (1960) when she gets killed in the shower scene, is used.
The killing of the reporter was difficult to shoot, because a real knife was used albeit nobody got hurt, Juan was a little bit worried. But it was the editing and the post-production which made the scene so brutal; the actress couldn't take the scene at the premier of the movie, and she left the auditorium when her death came onscreen.
The entire movie was shot in Madrid, nothing in Boston because there was no budget for second units. Some USA locations were recycled from SUPERSONIC MAN stock footage, another movie that Juan had directed.
At various times there was an interest in doing a sequel following the film's financial success in the United States and Spain, but it was Juan that had already been involved in another project and tried to get a sequel started years later, but he couldn't. With show business the American producers soon forgot the idea, despite the success at the box office.
Before working on the movie, Juan first received a thirty page treatment of "Jigsaw" which was supposed to be a made-for-TV movie. He took the basic idea and wrote the screenplay, limiting only by the budget possibilities. During the shooting, dialogue and scene were improvised because the screenplay was too short and Juan also wanted to improve the story to take advantage of some locations.
The trivia items below may give away important plot points.
Before the fifth victim gets killed, she's so terrified that she visibly urinates her sweatpants; this happened for real on set from a real chainsaw being wielded a few centimeters away from the actress. Juan, liked the reaction that the girl expressed and left the accident in the final film.
Although it's revealed that the Dean is the killer in the film, Edmund didn't play the Shadow character; it was a double who was dressed in the killer costume. The hands were actually of the director's, who stated he used to do insert shots like that because it was easier than wasting time in explanations to another person.
When Kendall gets castrated in the end by the corpse, Sera was dressed with a protection and the hand had razors to cut the actor's pants open; this was difficult as the director stated (worrying about anyone getting hurt) because the actor got a little cut in his groin.
Originally there was supposed to be a love scene between Mary and Kendall according to the director, but Linda Day declined; so the scene with the two at the apartment, Kendall asks to come in, but Mary turns down his offer.