During Ellen Burstyn's impassioned monologue about how it feels to be old, cinematographer Matthew Libatique accidentally let the camera drift off-target. When director Darren Aronofsky called "cut" and confronted him about it, he realized the reason Libatique had let the camera drift was because he had been crying during the take and fogged up the camera's eyepiece. This was the take used in the final print.
When Ellen Burstyn first read the script offered by Darren Aronofsky, she was horrified by it and rejected the role. It was not until after she watched a video of Pi (1998) Aronofsky's previous film - that she changed her mind and accepted the role.
Darren Aronofsky shot this film like a hip-hop montage (a sequence extremely short shots), to get the sense of overwhelming addiction and loss of control. An average film contain 600 to 700 cuts, but this film contains over 2,000.
The Tappy Tibbons material was shot in one day, with Christopher McDonald improvising a good deal of his material. At the end, the SAG extras for the audience and the crew all gave him a standing ovation.
In addition to having a camera mounted to her for certain sequences, Ellen Burstyn, spent four hours every morning being fitted with prosthetics, wearing four different necks (both fat and emaciated), two different fat suits (a 40-pound and 20-pound suit), and nine different wigs.
The "Lux Aeterna" portion of the music composed by Clint Mansell for the Requiem for a Dream soundtrack has often been used since in many other contexts, such as trailers for other films (including The Lord of the Rings: The Two Towers, The Da Vinci Code, I am Legend, Man on Fire, Sunshine, etc.), soundtracks for video games (Total Miner: Forge, Assassin's Creed), and background music for TV programs and advertisements. It is also commonly used as promotional and/or entrance music for many different college and professional sports teams, including: the Notre Dame, Missouri State University, Boston Celtics, and Virginia Commonwealth University basketball teams; the University of Alabama and Bowdoin College football teams, and the England Rugby Union Team.
The man peeling the orange (and the orange truck) in the scene where the characters go to receive a new shipment of drugs not only indicates their next destination - Florida - but also serves as a nod to the Godfather films, where the presence of oranges indicated disaster.
The overhead shot of Marion in the bathtub followed by her screaming underwater was an exact replica of a scene in the Japanese animated thriller, Perfect Blue (1997). Darren Aronofsky secured the rights to a live-action version of the film to enable including this scene in this movie.
In the opening scene you can hear the sounds of a string quartet tuning up for a performance in the soundtrack. Just before the title rolls down, you hear a conductor tap on his music stand to ready the quartet for a performance. The people tuning up are "The Kronos Quartet", who played most of the music for the film. The maestro bringing them to attention is Darren Aronofsky, the director.
The pills Sarah Goldfarb takes in the film are actually Synthroid pills taken for thyroid hormone replacement for people with hypothyroidism; this is also true for the pills Marion, Tyrone and Harry take before the party.
During one scene in which Sarah is hallucinating, her entire apartment is taken apart piece by piece as though it was the set of a television show. Several crewmembers of the mock television show pass Ellen Burstyn in her chair, including a man carrying a clip board with the Greek letter/mathematical symbol "Pi" on the back - the title of Darren Aronofsky's first film.
In the scene that Marion calls Big Tim, there is a shot of his phone number on a piece of paper. It was not the standard "555-" number used in movies. They were able to get away with this because only six digits of the phone number are visible. However if you listen carefully, Tyrone does speak all seven digits. But, after the scene where Harry is with the doctor, Marion turns over the photograph of her and Harry, revealing the full telephone number.
While Tappy Tibbons mentions several times that there are three steps to his program, only two steps are ever mentioned in the film; Tappy tells his audience to avoid red meat and to avoid sugar, but Sara is always prevented from hearing the third step. Director Darren Aronofsky had intended the third step to be the removal of pharmaceuticals from the equation. The film's producers asked Aronofsky to change the third step because they were concerned that pharmaceutical companies would feel that the film's message was targeting them or encouraging people to avoid taking medication. Instead of coming up with a new third step, Aronofsky edited the footage so that Sara would either start to daydream or feel the effects of the drugs and be unable to hear the third step. In supplementary material for the film, the third thing that drives most people crazy was changed to "no orgasms."
Darren Aronofsky described this film as exploring different types of addiction: "The Harry-Tyrone-Marion story is a very traditional heroin story. But putting it side by side with the Sara story, we suddenly say, 'Oh, my God, what is a drug?' The idea that the same inner monologue goes through a person's head when they're trying to quit drugs and cigarettes, as when they're trying to not eat food so they can lose 20 pounds, was really fascinating to me."
Darren Aronofsky initially wanted the three main heroin addicts featured in the film (Harry, Marion, and Tyrone) to be much younger than they were in Hubert Selby Jr.'s novel and screenplay. Aronofsky felt that changing the principal characters' ages to around 14 to 16 would further demonstrate the devastating impact of drugs and lead to a greater emotional reaction from the audience. Selby agreed with the director, however the film's producers felt that American audiences would find the film too horrific if young teenagers were to experience such awful events. Selby and Aronofsky lobbied for the characters to be made younger, but producers argued that the film could not be shown in theaters if the protagonists were made out to be teens. Selby acquiesced to the point, but Aronofsky only reluctantly agreed to continue the project with the older characters.
The four main characters - Sara and Harry Goldfarb, Marion Silver, and Tyrone Love - are significantly named, as they turn to drugs in order to gain a better life: Sara wants to be glamorous (gold), Marion and Harry want their own business (gold/silver), and Tyrone wants a better home than what his mother provided for him (love).
Hubert Selby Jr. wrote the novel in 1978, when medical facilities were inadequate and often abusive and uncaring of their patients. This film doesn't mention the era it's set in, but it can be thought to be in the 1970s; ironically, the film was criticized for showing medical institutions in a bad light.
When Tyrone and Harry are trying to get the drug shipment from Florida, one of the dealers is shown peeling an orange. This is a reference to The Godfather (1972) trilogy, where the use of oranges foreshadows something bad about to happen. They don't get their drugs, and things get worse from then on.