The film's pre-production was only five days; it was shot and cut at the same time in eighteen days (all of the bathroom scenes were shot in six days). The actors had absolutely no rehearsals. The rehearsal takes were actual footage for the film.
Director James Wan wanted the camera movements to reflect the two main characters' emotions and personality. He filmed Dr. Gordon with steady controlled shots and Adam as handheld shots to capture their emotions of the situation.
The film's screenplay was written in 2001 as a calling card for director James Wan and Leigh Whannell trying to break into Hollywood. They shot a low-budget short based on a scene in the film, and this proved successful enough to attract the attention of Evolution Entertainment. They immediately formed a horror genre arm called Twisted Pictures and gave Wan and Whannell a small budget.
Directors James Wan and Leigh Whannell wanted to make a film after they finished film school, but they could only afford one room. However, they challenged themselves to create a film that only occured in one room. "Saw" was the product, and it was considered one of the most profitable and successful horror films of all time.
Shots cut in the R-rated version, according to directors James Wan and Leigh Whannell, included ones of Amanda sifting through the intestines, Paul struggling through the razor wire, and some forensic ones. The color was made more even and the sound was altered because the MPAA had problems with the tone of the original cut shown in Sundance.
In post-production, director James Wan discovered that he did not have enough shots or takes to fill out most of his scenes, so he and editor Kevin Greutert created their own filler shots by doctoring some of them to make them look as if they were filmed through a surveillance camera.
The scene in which Dr. Gordon turns off the lights and whispers to Adam was written differently in the script. The characters were to cut open opposite ends of a long pipe with their hacksaws and speak through it. This sequence was actually shot but later cut, because director James Wan decided that the characters being able to cut through a pipe made no sense if they could not cut through their chains.
The film contains many references to the films of Italian horror and giallo director Dario Argento. The creepy painted puppet is a reference to Argento's Deep Red (1975), while the unseen killer's black gloves are one of Argento's trademarks and can be seen in almost all of his films.
According to the DVD commentary, casting agent Amy Lippens asked director James Wan who he wanted to play the character of Amanda. On a whim, Wan suggested Shawnee Smith, on whom he had had a crush since his teen years. He was surprised when Lippens came back a few days later and told him that they had secured her for the role.
To help sell his concept for the film, Leigh Whannell shot a scene in which he appears to be ensnared in a bear trap. There were no visual effects involved and he actually had to place the teeth of the rusty bear trap in his mouth to make it seem real.
Director James Wan and editor Kevin Greutert had trouble editing the film, as they did not have enough shots they wanted and had difficulties editing some scenes, including Dr. Gordon listening to his tape and the detectives in Dr. Gordon's office, the first time they meet.
Cary Elwes filed a lawsuit against the producers and the production company because he only received a nominal salary with backend revenues. He claimed to have been promised 1% of the profit, which would be considerable since "Saw" (2004) went on to earn over $100 million globally. The case was finally settled out of court, but mostly as a result of the disagreements, and Elwes chose not to be involved with any of the sequels until Saw 3D: The Final Chapter (2010).
Director James Wan and Leigh Whannell wrote the script and submitted it to their manager. The manager then sent it to an agent in Los Angeles, who summoned the two of them for a meeting. They were encouraged to shoot a scene from the script as a short film, which they started passing around to studios.
In the original draft, Zep (Michael Emerson) was to do strange acts with Alison's underwear in her drawers, but Leigh Whannell cut it out as he thought it was a bit "too far," and it was re-written, instead Zep held a gun to Allison's head while he listened to her heart.
Detective Kerry states that Jigsaw technically isn't a murderer, that he finds ways for his victims to kill themselves. She is right, he technically never killed anyone himself; however he could have still been charged with second degree murder for each of his victims that died. In the United States there is a statute that allows someone to be charged with murder even if they didn't actually intend to kill someone. Normally if someone's reckless actions unintentionally cause a person's death they are charged with involuntary manslaughter. However, when someone's actions go beyond reckless and they do something that puts another person in a situation where their death is a likely outcome they are doing what is known as "showing a depraved indifference to human life". It's called depraved indifference murder; by putting his victims in a situation where the likely outcome will be their death Jigsaw would have been charged with multiple counts of second degree murder under that statute if he had been apprehended.
According to the official "Saw" website, in a still image used in one of the flash scenes, Adam's last name is "Faulkner." However, it was revealed in Saw V (2008) that Adam's last name was "Stanheight", as indicated by a document observed by Agent Strahm.
According to James Wan, the construction of Billy the Puppet's face for "Saw" involved clay, papier-mâché, and black ping pong balls with the irises painted in for the eyes. Paper towel rolls were used internally. To make him move, the puppeteers pulled him along on a fishing line. For "Saw II", Billy was redesigned with mechanical features that could be manipulated by a remote control, including his unhinged jaw and his eyes. For "Saw III", the prop crew was given the original puppet, but found it unfit for work, as time had damaged it. Instead, they recreated Billy, using waterjet-cut foam for his body instead of fiberglass, equipped with plates to hold the puppet together and magnets to attach him to his tricycle. The back of his head was removable, to make it easier to slide animatronic pieces in. For "Saw IV", the prop crew again made the body of waterjet-cut foam, held together by metal rods. They used strong magnets to make a flat rear for the puppet, so it could be easily positioned on any metal surface. The resin-filled ping-pong ball eyes were moved with a remote control, as was the mouth.
Two of the three actors given main billing (Cary Elwes, Danny Glover, and Monica Potter on the DVD case) are "Alex Cross" film alums. Cary Elwes appeared in "Kiss the Girls" (1997) and Monica Potter appeared in "Along Came a Spider" (2001).
The trivia items below may give away important plot points.
The identity of the Jigsaw Killer is revealed early in the film, during a flashback. When Zep (Michael Emerson) says, "He's a very interesting person", on John's hospital bedside table in front of him is a diagram of "The Reverse Bear Trap".
At the beginning of the film, Adam (Leigh Whannell) is informed via the cassette tape, that he must free himself from his chain, and escape the bathroom. Lawrence (Cary Elwes) is informed that if he does not complete his game, then he will be left to die in the bathroom. The opposite fates befall both characters at the end of the film.
When Dr. Gordon begins sawing his foot off, Adam is heard screaming and crying in the background. His screaming and crying is the same one used from the very last scene during the final climax. The audio was edited and re-looped.
Leigh Whannell came up with idea of giving John Kramer a brain tumor while spending time in a neurology ward for anxiety and headaches. He said that undergoing all kinds of tests and expecting bad news made him reflect on his own mortality. He used this experience in creating a character who still had only a few years to live.
Instead of "The Quadruple Shotgun Trap", the creators wanted a trap that consisted of two things that would spring from the walls and snap shut on Sing (Ken Leung) and fold him up into a box. They described it as an "iron cocoon." They decided against the idea, as it would have cost too much money for visual effects. The iron cocoon idea, however, was finally used in Saw 3D: The Final Chapter (2010).
"The Reverse Bear Trap" was also used in the original "Saw" short film, attached to a character named David. The scenario played out the same as the later version, with David escaping from the contraption a few seconds before it went off.
It can be argued that John had intended for Adam to be able to escape from the room, but by the actions of Amanda, he was prevented from it. In "Saw III", it is revealed that Amanda was the one who threw the key on his chest, instead of putting it in his pocket, hinting that she knew he would lose it.
Although "Hello Zepp" is considered one of the most famous scores in the film, the line is never actually uttered throughout the film. The misconception is mostly due to the fact, that this is the title of a song in the soundtrack, and is played during the final revelation scene. This piece of music became the franchise trademark, and was used as the score for the final scenes, which included plot twists and a rapid montage of flashbacks. However, the real line is "Hello Mr. Hindle, or as they called you around the hospital: Zep."
Directors James Wan and Leigh Whannell returned for several re-shoots, but the original actors were unavailable by that time. Whannell played the parts himself and Wan used close-up shots of the characters' bodies, avoiding showing their faces. Whannell played Detective Sing (Ken Leung) entering the building with a shotgun and the body of Sing falling down after being shot. Also, the close-up shots of Amanda's (Shawnee Smith's) hands in her torture and murder scene were Whannell's, and he wore a wig to make his shadow on the wall appear more like Shawnee's.
Not including the video game, Detective Tapp is actually the only main protagonist in the feature film franchise who does not die from either Jigsaw's traps or playing one of his games. He is rather shot instead and left for dead in the sewers for interfering.
As Lawrence is loading the cartridge into the revolver to shoot Adam, the camera briefly shows all six chambers of the cylinder to be empty. Revolvers don't eject spent cartridges, yet the man on the floor was supposed to have shot himself with the same gun. What looked to be a simple prop error is actually a subtle foreshadowing to the twist ending.
This is the only "Saw" film where the two main characters are not in a trap but instead held captive. Compared to other characters, they are not in a trap (such as "The Reverse Bear Trap" and "The Razor Wire Maze"). instead, they are held captive in a bathroom.
Zep (Michael Emerson) claims that the patient, John, who is later revealed to be Jigsaw, is a "special" person. In the TV series "Lost" (2004), he also claims another person named John (John Locke) is a "special" person.