Mort looking head on into the mirror and seeing the reflection of the back of his head is a reference to the paintings of Belgian surrealist painter René Magritte, such as "La Reproduction Interdite" (English: "Reproduction Prohibited"). He often depicted a mysterious man in a bowler hat not unlike the hat that Shooter wears.
In the scene where Johnny Depp bursts in on Maria Bello and Timothy Hutton in the motel, David Koepp wanted Bello and Hutton to look shocked and scared. He made them both lie in the bed for fifteen minutes before Depp rushed in. The production crew set up large speakers that blared static noise when the script called for them to be scared. The lights in the room were also rigged to go on when Johnny Depp opened the door, startling the actors further. No one knew exactly how to act.
Johnny Depp's character's name is Morton Rainey. Toward the end of the movie, this character purchases three items at the grocer's. One of the items is a box of MORTON's salt, whose motto is "When it rains it pours." Thus Morton Rainey.
In the magazine that should have Mort's story, the second story in the contents is called "The Long Walk." This is also the title of a book by Richard Bachman, who happens to be the pseudonym of Stephen King.
Mort explains to John Shooter, in his defense, that his story is the original and was published originally in the June 1995 issue of Ellery Queen's Mystery Magazine. Ted Milner (Timothy Hutton) tells Mort that he's from a town called Shooters Bay, Tennessee, adding doubt in Mort's mind about Ted's real intentions. Timothy Hutton's father, Jim Hutton, portrayed Ellery Queen (1975) in the 1975 television series. Ellery Queen was the son of a police detective, and the writer of mystery novels, as well as an amateur detective.
In the book "Secret Window Secret Garden" the titles written by John Shooter and Mort Rainy are the opposite way around to how they are shown in the film. Mort Rainy's book was called "Sowing Season" and John Shooters book was "Secret Window, Secret Garden"
This film, which co-stars Timothy Hutton, involves a story published in the Ellery Queen Mystery Magazine. Hutton's father, Jim Hutton, starred as Ellery Queen in the television series Ellery Queen (1975).
The trivia items below may give away important plot points.
When Mort's conscience is revealing the real situation to him at the end of the movie, Mort begins to yell "Rah! Rah! Rah!" Johnny Depp took this from his son, who was pre-verbal and expressed his confusion by saying "Rah!" Depp felt that Mort would be so confused at that point that he would not know what to say to convey what was happening, and that he would become pre-verbal again.
At a moment when suppressed memories of Morton Rainey's come to the surface, he looks in the mirror and hallucinates that his reflection is facing away from him. Under similar circumstances, the same thing happens to the character of Selena St. George in Dolores Claiborne (1995), another Stephen King adaptation...but curiously enough, neither book contains such a scene and the two movies were adapted by different writers.
In the scene where Morton is lurking around the house after hearing a strange noise, he is stopped in his bedroom, where he thinks John Shooter is in his bathroom. He then runs in swinging a fireplace stick and breaks his mirror. He then realizes his reflection was what he saw in the bathroom, hence a foreshadow to the end of the film, where it's revealed it was all in his head, and that John Shooter was a character Morton created.
This is the second film adapted from a Stephen King work in which Timothy Hutton features. Coincidentally, both films are about a writer tormented by his alter ego. The first was The Dark Half (1993), where Hutton played the dual roles of the writer, as well as his alter ego.