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).

Christian Bale found that his weakened condition caused problems in the more demanding action scenes. He found running a particular problem as he simply had no leg muscles left.

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 conditions he subjected himself to. He was also able to deal with the very hot July and August temperatures in Barcelona much better than the rest of the crew.

Christian Bale took up smoking to help him curb his appetite when he was crash-dieting.

Brad Anderson and his screenwriter Scott Kosar were turned down by virtually every American studio and producer they approached on the grounds that the screenplay was "too weird".

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".

Staying awake for a year, as mentioned in the film, is physically impossible for a human being. The actual longest time someone is recorded to have stayed awake for is 11 days, 25 minutes. This was set by 17 year old San Diego student Randy Gardner in 1964 for a Stanford University experiment.

In one scene Trevor falls asleep while reading Fyodor Dostoevsky's 'The Idiot'. Two of the characters in that book are Marie and Nicholas.

Scott Kosar wrote this screenplay on spec straight out of film school. He then touted it around all the usual Hollywood studios for several years. The writing was well thought of, but the overwhelmingly dark mood was not, and financing could not be secured in America.

Writer Scott Kosar loves Nine Inch Nails, so much so that 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 also wanted an NIN soundtrack for the film, but Brad Anderson vetoed this. Ironically, Reznor, along with writing partner Atticus Ross, later became an Oscar winning composer for film soundtracks.

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.

Writer Scott Kroopf refers to the script as "the last movie that Alfred Hitchcock would have ever made".

There were very few CGI effects in the film. The first café scene had a CGI inserted plane taking off in the background.

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.

Brad Anderson shot most of the film either on crutches (he had torn his Achilles tendon) or from a gurney (his back gave out on him).

Roque Baños' score is a deliberate homage to the work of Bernard Herrmann, Alfred Hitchcock's favorite composer. There is also a nod to Miklós Rózsa in the use of the theremin which Rozsa employed to Oscar-winning effect for his score to Hitchcock's Spellbound (1945) in 1945.

The first film Brad Anderson has made that's not based on one of his own screenplays.

Brad Anderson wanted his film to have the same kind of visual expressionistic feel as The Cabinet of Dr. Caligari (1920), Nosferatu (1922), Vampyr (1932) and I Walked with a Zombie (1943).

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.

When Trevor drives off into the countryside, it's filmed in the Principality of Andorra, a landlocked independent country between Spain and France.

In the subway you can see a poster of the film Rottweiler (2004), another film produced by Julio Fernández.

This film contained two actors who have played Batman. Christian Bale played Batman in Batman Begins (2005), The Dark Knight (2008), and The Dark Knight Rises (2012). Michael Ironside voiced Batman in the "Dark Knight Returns" segment of The New Batman Adventures: Legends of the Dark Knight (1998).

Christian Bale and Michael Ironside co-star alongside each other in Terminator Salvation (2009).

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 same line is said in Fight Club (1999), another film about a man who suffers from terrible insomnia, which results in memory loss and schizophrenia.

Ivan's license plate number - 743CRN - is the reverse of Trevor's number - NRC347 (seen at 1:06:36).

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 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.

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; Reznik blinks only his left eye in a few scenes; 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.

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.

Miller says to Reznik, "I need a hand." About a minute and a half later, Miller loses his hand in a machine.

Reznik means "butcher" in Czech language.

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.

Trevor's apartment and that of Marie's seem very similar, with only the furniture and dressings moved around. This is for both practical cost and plot reasons.

In this film as well as American Psycho (2000), both characters portrayed by Christian Bale suffer from delusions and hallucinations, some of which are explicitly stated or implied to make up large sections of each film. 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.

When Reznick suspects Miller as the stalker, he enters Miller's name into the hangman riddle; once the letters are filled in, like Miller the hangman is missing one arm.

This isn't the first time Michael Ironside's character has lost an arm (or two). In Total Recall (1990) he loses both while fighting on an elevator with Arnold Schwarzenegger's character. In Starship Troopers (1997), he plays a battle-scarred soldier with a robotic arm.

Marie's running after Nicolas' seizure inside Route 666 is the same as when Nicolas was hit by Trevor's car.

In the amusement park ride, when the cardboard man with the sign pops out and says "STOP!", another cardboard cutout of a kid pops out and the tram bumps into it, with the accompanying sound effect of the kid screaming. This is an extremely subtle nod to Trevor's climactic accident.

26 minutes into the movie, Trevor looks up at the tall tube-shaped water tower and the scene fades to show Maria in the coffee shop with Trevor. This provides a small hint about Maria's identity, in the fact that Trevor only saw her by the water tower.

A bumper sticker on the front bumper of Trevor's truck has the words, "I'd rather be fishing" in the scene where Trevor is hit by a car outside of the DMV.

A fish's head is mounted in the "Boiler Room" bar.