Trivia: In the scene that Amanda digs through her cellmate's innards for a key, the hands shown in the close-up shot are not actually Shawnee Smith's hands at all. They're Leigh Whannell's hands with his nails painted black. Likewise, several shots of Detective Sing (when he first enters Jigsaw's lair and when he is killed by the hidden shotguns) it is not actually Ken Leung but Whannell dressed as Leung's character.
Trivia: As mentioned on the DVD commentary, Amy Lippens, the casting agent, asked James Wan, the director, which actress he's interested in for the role of Amanda. Wan suggested Shawnee Smith, whom he had a crush on since he was a teenager. A few days later, when Amy Lippens told him that they actually signed Shawnee, he was happily surprised.
Trivia: The scene in which 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 couldn't cut through their chains.
Trivia: Pre-production for this movie lasted only five days. The bathroom scenes were shot in just six days and the actors' rehearsal takes were used as the actual footage for the film.
Trivia: The director and writer were also the director and writer of a low budget Australian film called Stygian. There is even an inside joke when someone at one point refers to "Stygian Street" and you see the street sign.
Trivia: The music that starts to play at the end, which has become a signature tune for the twists at the end of the "Saw" films, is titled "Hello Zepp". Though the tape Adam finds actually adresses "Hello Mr. Hindle".
Trivia: Danny Glover's wall has hundreds of newspaper clippings attached to it. At one point in the film, we are shown some of the headlines, i.e. "Psychopath teaches sick life lessons." If you actually read the text below you'll find that it bears no relevance to the headlines whatsoever. (You may need to use the zoom facility). Two of them are about a fourteen year old boy who died in an explosion, due to "Goofy Clay", and if you pause and read the wording, they have exactly the same text. Not really a mistake due to needing pause/zoom, but interesting to look for.
Trivia: The doll/puppet seen in the film (called "Billy" behind the scenes) was actually built by director James Wan, and is the exact same doll used in the original short film that was made to help sell the script. (Although Wan did slightly modify it between the production of the short film and the production of the feature film.) Due to the tight budget, the producers told Wan that they didn't want to spend unneeded money making a new doll when the original was still available. Wan later admitted he was slightly annoyed when the sequels were released, as each subsequent sequel had the budget to allow for the construction of newer, more intricate versions of the doll.
Trivia: When the lights are turned off in the bathroom, and the glow-in-the-dark "X" is revealed, the effect wasn't actually achieved using glow-in-the-dark paint. Instead, the lights were dimmed somewhat on-set and a bright light equipped with an "X"-shaped stencil was projected onto the wall. This is because there was too much light on-set for glow-in-the-dark paint to be visible, as even during dark scenes, film sets require at least a moderate amount of light.
Trivia: As is often the case with micro-budget films, the movie was not shot widescreen, but rather was shot on cheaper 4:3 full-frame film stock and cropped for the widescreen theatrical release. Thus director James Wan filmed the movie ahead of time with the knowledge that the tops and bottoms of the frame would be missing from the theatrical cut, and he made sure to compose the shots accordingly. Unfortunately, instead of panning-and-scanning the cropped widescreen release for the full-frame home-video release, the distributors merely uncropped the image. This causes some rather strange and subtle blunders in some full-frame home-video releases, as portions of the frame were visible that shouldn't have been.