The producers of the film claim that Christian Bale dropped from about 173 pounds in weight down to about 110 pounds in weight to make this film. They also claim that Bale actually wanted to drop down to 100 pounds, but that they would not let him go below 120 out of fear that his health could be in too much danger if he did. His diet consisted of one can of tuna and an apple per day. His 63-pound weight loss is said to be a record for any actor for a movie role. He regained the weight in time for his role in Batman Begins (2005).
Brad Anderson had not asked Christian Bale to lose such a vast amount of weight and was genuinely shocked when he saw Bale's appearance on the first day of shooting. He also confessed to being thrilled by the actor's dedication.
Oddly enough, Christian Bale claimed that he actually enjoyed losing so much weight, oftentimes finding himself in a euphoric state due to the extreme condition he subjected himself to. As well he was able to deal with the very hot July and August temperatures in Barcelona much better than the rest of the crew.
When Trevor is driving Marie and Nicholas back home, if you look closely through the driver's side window when Trevor stops, you can see two people sitting in the background. According to Brad Anderson, they were junkies who were actually shooting up heroin during the filming of the scene.
Because the film was shot almost entirely on location in Barcelona, Spain, the production designers had to painstakingly re-create the look and feel of Los Angeles with many "everyday Americana" items in both interior AND exterior shoots. This included cigarettes, alcohol, street signs, traffic signals, public telephones, retail signage, automobiles, license plates, and also shooting in what writer Scott Kosar called "some of the worst areas of Barcelona".
Writer Scott Kosar loves 'Nine Inch Nails', and the movie's protagonist Trevor Reznik is named for NIN frontman Trent Reznor. Kossar's original script had a quote from their lyrics on the first page. He wanted a NIN soundtrack for the film, but Brad Anderson vetoed this.
Scott Kosar wrote this screenplay on spec straight out of film school. He then touted it round all the usual Hollywood studios for several years. The writing was well thought of, but the overwhelming dark mood was not, and financing could not be secured in America.
Although the film has a Hitchcockian feel to it, the decision to make it like that was only really consciously taken in the editing suite when they started adding some musical cues. The idea was to give the film a Bernard Herrmann like score, Herrmann being one of Alfred Hitchcock's main composers.
The Spanish producers wanted to use local Spanish talent in minor roles and then dub them into English. Brad Anderson resisted this suggestion as they thought the dubbing may not go over well, and insisted on casting local expatriate American and British actors instead who could speak with an American accent.
Trevor's landlady is named Mrs. Shrike. This is a reference to Nathanael West's novella "Miss Lonelyhearts" in which the main character suffers a spiritual dilemma and is antagonized demoniacally by Shrike, his editor.
The trivia items below may give away important plot points.
The time of 1:30 AM is significant throughout the movie. Trevor often notices something out of the ordinary at this time. During the 1 hour 30 minute mark in the movie, the major plot twist is revealed.
At one point Trevor says, "Nobody has ever died from insomnia." The exact same line is said in Fight Club (1999), another film about a man who suffers from terrible insomnia which results in memory loss and mistaken identity.
Many references to "sinister" in both the modern sense ("ominous/evil") and the ancient sense ("left/unlucky"): Both Miller and Ivan have damaged left hands; Ivan is standing on the left in the photo; Ivan's left eye is a bright blue while the other is dark brown; Marie's kitchen is the "last door on the left"; Stevie's bruise is on the left side of her face; the hangman game progresses from right to left; and the lye or bleach is always to the left of whatever sink Trevor uses. On three occasions, Trevor encounters a "fork" in the road: in the "Route 666" ride, in the sewer, and when traveling to the airport ("Airport" vs "Downtown") - the "left" (sinister) path is always darker and more dangerous, and each time Trevor chooses to follow it, except for the last time, when he chooses right (downtown, "salvation").
As Trevor and Nicholas travel through the Route 666 amusement ride and turn left onto the "Highway to Hell", various odd but later significant scenes are momentarily sliced in with the strobe lights, but those scenes are only noticeable on a frame by frame view.
When Trevor is inside Stevie's apartment for the final time, she asks how he got so bruised. Trevor mentions that he was the victim of a hit-and-run, to which she responds that such people should be hanged. Trevor already received multiple threatening notes with a Hangman game on them, another clue to the final twist.
While Trevor is riding Route 666, one shot reveals a sign overhead that reads "Crime and Punishment", the Fyodor Dostoevsky novel about a guilty man who attempts to justify his crime. This also provides the bookend, if you will, to Trevor's home reading, The Idiot, which is Dostoevsky's examination of an innocent man who others lure into their webs of corruption and which has a very surprising and climatic ending; thus foreshadowing the movie's own ending. The movie also has similarities to Dostoevsky's "The Double", which contains a character that appears and begins to ruin the protagonist's life. The doppelganger turns out to be a pseudo-schizophrenic manifestation of the protagonist's less desirable characteristics.
There are several fishing and fish references throughout the movie - significant clues about the plot which only become clear later on. These include: the fishing rod in Trevor's cupboard; the fish paintings on his kitchen wall; and the fish mounted in the bar.
When looking through the photo album Trevor sees a picture of himself in a child sized red car. That same car is in the hallway when he collapses outside Stevie's apartment after his "hit and run" - another foreshadow to the twist ending.
In this film as well as American Psycho (2000) with Christian Bale as the lead actor, both of the characters he portrayed suffered from delusions and hallucinations. Furthermore, one of Trevor Reznik's co-workers, Jones, is played by actor Reg E. Cathey, who also happened to play one of Patrick Bateman's many "victims" in American Psycho.
In the final scene, when warden showing him the way, Reznik wears a t-shirt that says "Justice Brothers" with a racing car on it, more clearly seen in the bedside photo of Reznik (Ivan) at 1hr 17min. Final images of T Shirt doesn't show the plural of Justice Brothers, only Justice Brother.