In the film, the works of Sutter Cane are occasionally quoted. Most if not all of these quotes are actually taken directly from several H.P. Lovecraft short stories with some adaptations to fit them into the film story. Most notably, in the scene where Styles reads to Trent as he gazes into the abyss--her speech lifts much of its description, including such elements as "the illimitable gulf of the unknown" from the last few paragraphs of Lovecraft's "The Rats in the Walls." In an earlier scene as well, Trent reads a line verbatim from Lovecraft's "The Haunter of the Dark," in reference to the black church being "the seat of an evil older than mankind and wider than the known universe."
The building used as the mental institution at the beginning of the film is actually a water filtration plant in the Beaches area of Toronto. It has been the scene of other movies, including the island fortress in Undercover Brother (2002). Filming can no longer take place inside this building following the September 11 terrorist attacks on the USA.
The Sutter Cane character is clearly based on John Carpenter's friend Stephen King, even referencing King's New England roots, with Hobb's End filling in for King's Castle Rock. Carpenter directed a film version of King's Christine (1983). Additionally Michael De Luca had previously written the screenplay for the King adaptation The Lawnmower Man (1987). However, the characters say that Sutter Cane is even more popular than Stephen King.
The effect of Julie Carmen spinning her head around was done by having a contortionist stunt-double wear an up-side down prosthetic mask of Carmen's face. Since the stunt double couldn't see, the filmmaker had to guide her on where to go by sound.
References to the "Old Ones" by Sutter Cane, along with certain representations of monsters printed on his books and latter brought to life, are allusions to The Cthulhu Mythos - a series of stories by H.P. Lovecraft and continued by other writers into modern times. In this sense, one could consider "In the Mouth of Madness" John Carpenter's own contribution to The Cthulhu Mythos.
The "Wall Of Monsters" at the end was not several monsters being controlled individually. Instead, it was one single special effect that was attached to a vehicle-like wheelbase and had to be pushed along with a crank. During the filming of this scene, the Wall of Monsters accidentally ran over Greg Nicotero's foot and he had to be taken to a nearby hospital.
The six Sutter Cane novels which Trent uses to track down the town of Hobb's End are the following: "The Hobb's End Horror", "The Feeding", "The Whisperer in the Dark", "Something in the Cellar", "The Breathing Tunnel" and "Haunter Out of Time". Also, these titles are direct references to H.P. Lovecraft's works, a direct inspiration for the movie. These are "The Dunwich Horror" "The Whisperer in Darkness", "The Rats in the Walls", "The Thing in the Doorstep", "The Shadow Out of Time" and "the Haunter of the Dark". Also, the title of the movie refers to "At the Mountains of Madness", yet another work from Lovecraft's.
Stephen King's short story 'Crouch End' was published in the 1980 anthology 'New Tales of the Cthulhu Mythos', with a reworked version later appearing in King's 1993 anthology 'Nightmares and Dreamscapes'. In the story, a man and woman visit an English suburb called Crouch End, where they become stranded and are unable to leave. They are taunted, tormented and chased by a pair of deformed, otherworldly children, who later summon a Lovecraftian Old God. Some aspects of King's Crouch End bear great resemblance to the film's town of Hobb's End.
Although Sutter Cane might appear to be representative of Stephen King, he's more likely to be based on H. P. Lovecraft, and Hobb's End is based on Lovecraft's Arkham rather than King's Castle Rock. The plot of the film also more closely resembles something Lovecraft might have written, focussing on the effects that horrific occurrences have on one's sanity, rather than on the external horrors that most of King's writings lean towards. Even the title, "In the Mouth of Madness," appears to be an homage to Lovecraft and his story, "At the Mountains of Madness."
John Carpenter has a brief cameo in the film as an asylum patient in the beginning of the movie. When Trent is shouting "I'm not insane!" throughout the asylum, the camera pans past Carpenter, who shouts "me neither!".
Despite being set in and with heavy reference to New York, Sam Neill speaks with his natural New Zealand accent, most notably when he says the word "them" in the opening of the film, emphasizing the "e" as an "I".
After Sutter Cane says "Did I ever tell you my favorite color is blue?" It is realized that throughout the entire movie, whenever an actor has a close up, their eyes are blue, proving Sutter Cane's power.
When Jürgen Prochnow lets the monsters from the other side into our world, originally in the script the entire town was sucked into the other side. When this proved to be too costly, an effects artist over at Industrial Light and Magic recommended that instead he "tears" himself apart like paper.
At the end of the film, when Sam Neill approaches the movie theater showing the meta-film, the following credits can be seen (though some names are slightly blurred) on the movie poster outside the theater: "New Line Cinema Presents a John Carpenter Film 'In the Mouth of Madness' Starring John Trent, Linda Styles, Jackson Harglow, Written by 'Michael De Luca', Associate Producer Artist W. Robinson, Director of Photography Gary B. Kibbe, Production Designer Jeff Ginn, Produced by Sandy King, Director John Carpenter." Except for the three lead character names, all the other credits are for the real film's actual crew.