Actor David Caruso reports in the official Production Notes that he saw "something pass my window" when shooting inside the Bonner Medical Building of Danvers State Hospital. "I didn't want to tell anybody, because people would start looking at me strangely..."
This was one of the first feature films shot using Sony's 24P HD video, which shoots at 24 frames per second, like film, as opposed to the 30 frames per second of conventional NTSC video. Using this technology, Brad Anderson and director of photography Uta Briesewitz were able to produce the uniquely effective, deep-focus images using mostly natural light.
Only 3 rooms had additions outside of the natural setting for atmosphere, the kitchen had meat hooks hung, the tunnel had plastic surgical gloves hung up, and the hydrotherapy room had a metal tub added. Almost everything else related to the asylum setting was found on site as the crew scoured the building for set dressing to keep things authentic.
The movie is mentioned in the book Project 17 by Laurie Faria Stolarz. The book takes place one night in Danvers State Hospital. A group of teens spend the night to film a movie, and encounter supernatural events - much like the psychological aspect of this movie. The main character in the book references the group's actions as reminding him of Brad Anderson's movie Session 9.
The fictional "Patricia Willard scandal" at Danvers State Hospital, cited by Mike at the film's beginning, strongly appears to have been inspired by a real-life wave of problematic "Satanism and sex-abuse" allegations that swept the United States circa the 1980s, including (among others) one involving the Amirault family in nearby Malden, Massachusetts. Reporter Dorothy Rabinowitz won a Pulitzer Prize for her book chronicling that bizarre case, "No Crueler Tyrannies: Accusation, False Witness, and Other Terrors of Our Times," in 2001.
The scene shot outside Danvers when Bill ('Paul Guilfoyle') and Gordon (Peter Mullan) are discussing the bid for the asbestos abatement job, apparently a student pilot was flying circles overhead for about 3 hours. This forced the talent and crew to film the sequence in spurts starting when the plane was furthest away and stopping as it grew nearer.
The trivia items below may give away important plot points.
A subplot was filmed involving a homeless woman who lives in the asylum. In the original ending, she witnesses all the murders and then kills Gordon. According to the DVD commentary, this subplot was removed because test audiences became confused, thinking she was actually Mary (the voice on the tape).
The shot in which you see Mike (Stephen Gevedon) walking through the gym from the balcony before his demise, was originally a long shot from the point of view from the homeless woman that was in the original subplot of the film that was cut. The full version of the scene is featured on the DVD's deleted scenes.