Robin Williams often played practical jokes in the middle of a take during the making of a film. According to producer Christine Vachon, Williams burst out of a doorway completely nude during one take for a chase scene.
After the film premiered at the Sundance Film Festival, Robin Williams overheard viewers say that they forgot it was Williams in the lead role 15 minutes into the film. An observation that made Williams proud.
Near the beginning of the movie, as the Yorkin family is going over their set of developed photos while in their house, they are eating Chinese food and inside of an opened fortune cookie it reads "Someone wants you to be happy". Director Mark Romanek actually searched through 30 fortune cookies before he found one that related to the film and used it in the scene.
Robin Williams was a fan of the Japanese anime series Shin Seiki Evangerion (1995) and specifically asked to have a model from the series incorporated as part of the film. The model itself, contrary to Jake's explanation that it is of a "good guy", is of an unintelligent, creature-like antagonist from the adult-oriented film ending to the series, Neon Genesis Evangelion: The End of Evangelion (1997). The boxed toy was supposedly from Williams' own personal collection.
Trent Reznor of 'Nine Inch Nails' was initially chosen by Mark Romanek to provide the musical score for the film. Romanek, however opted not to use the score Reznor provided. Some of the compositions meant to be used in this film were reworked and released on the album Still by Nine Inch Nails.
During the DVD commentary, director Mark Romanek stated that Jack Nicholson was up for the role of Sy before Robin Williams. This is not the first time Williams was up for a role first approached to Nicholson. When Nicholson was stalling to sign on as Joker in Batman (1989) Williams was in talks before casting was finalized.
In the first showing of the "photo wall", there are 854 photos. 29 high by 31 across minus three rows of 10, 14 and 21. At a later date the wall has 862 photos, same grid minus three rows of 2, 14 and 21.
Names of other photographers also appear, including: Yoshi Araki (Nobuyoshi Araki is a very well-known Japanese photographer), Bill Owens, 'Ellen von Unwerth' (a popular German photographer), Aaron Siskind, Paul Outerbridge.
When finding out what room Will is in at the hotel, Sy asks if he's in 217. 217 is famously the hotel room from Stephen King's novel The Shining (it's 237 in the film). Robin Williams was considered for the lead role in The Shining (1980).
The hotel where the abduction takes place, where Sy photographs the adulterers, is called "the Edgerton". This is a reference to the scientific engineer Harold Eugene "Doc" Edgerton, most famous for his pioneering work with high speed stroboscopic flash. This technique made it possible to photograph very high speed events, and some of Edgerton's most well-known images are of a bullet in flight cutting through a playing card, a bullet passing through an apple, a balloon bursting, and a drop of milk falling into a into a glass of milk, forming a perfect milk coronet.
During the scene in which Sy discusses an elderly customer who has him develop never-ending photos of nothing but her cats, the photos shown of each cat were photos that the cast and crew had brought in of their own pets according to the American Humane Society. It was seen as a way to get more natural-looking cat photos instead of staging shots with trained animals.
The trivia items below may give away important plot points.
Sy's rant about child pornography and Will Yorkin's betrayal of his family in the final scene, hints at Sy's backstory. In Sy's backstory, Sy had been exploited by his father for child pornography and the horrible evil and despicable things that his father forced him to do in front of the camera, left him with emotional and psychological trauma and discovering Will Yorkin family having an affair with Maya Burson, brought back painful memories of his own father and caused him to snap and go vigilante.
The number "86" appears on the wall as Sy is leaving the store after his last day on the job. When used as a verb, 86 is a slang term for refusing service or getting rid of something. First used as a "10" code for CB radio: 10-86 means death, finished. As "There was a 10-86 on the highway time to find another route. Traffic is backed up." Restaurants use "86" as a certain item is no longer available that it's "86 on the wasabi tuna"
At the end of the film, when he is being questioned by police, Sy Parrish (Robin Williams) tells Detective Van Der Zee (Eric La Salle) that he can tell that the detective is a decent man who'd never take disgusting pictures of his children or make them do things that children "should never have to do". This possibly explains or gives a bit of perspective on what demons may be causing Parrish's lack of social adjustment and mental fragility.