The film was shot in sequence, although the completed edit does not present the narrative in chronological order.
The film's title derives from the work of Dr Duncan MacDougall of Haverhill, Massachusetts, in the early twentieth century. MacDougall weighed people as they were dying in an attempt to demonstrate that the human soul was material and measurable. His experiment is widely regarded as flawed and unscientific.
The film was filmed almost entirely using hand-held cameras.
Naomi Watts agreed to take the part without reading the script beforehand.
In the scene where Jack Jordan attempts to hang himself, the overweight prisoner who rescues him bears a large tattoo on his chest reading "Maria Eladia". That name belongs to the wife of Alejandro G. Iñárritu, to whom the film is dedicated.
At the end, the Spanish-language dedication to María Eladia — "pues cuando ardió la pérdida, reverdecieron sus maizales," — literally reads "for when the loss burned, their cornfields became green again."
At the urging of Sean Penn, Naomi Watts actually struck him during a highly charged emotional scene between them.
The story centres on three characters who are, in various ways, believed to have confronted death. Paul Rivers (Sean Penn) and his wife appear to have accepted that his heart is failing and that he may not live long — until Cristina Peck's husband's death ultimately saves him; Cristina Peck (Naomi Watts) admits during a group therapy session near the start of the film that she 'ended up flat-lining at the hospital' but survived; and Jack Jordan (Benicio del Toro) is a born-again Christian, repenting for the sins of his former life as a criminal. In the film's closing monologue, Penn's character questions what effect death has on the human soul — 'how much is lost? [...] How much is gained?'. The death-related experiences these characters endure have clearly altered their personalities. The film, through both its title and its themes, questions the value and meaning of those changes.
In the scene in which Paul tells Cristina he had her husband’s heart transplanted into him, he says the operation took place on 11 October. 10 plus 11 adds up to 21.
When Paul discovers photographs of Cristina and her husband tucked inside a book, he is leafing through "Cruising Paradise", a collection of 40 stories about solitude and loss by playwright and actor Sam Shepard.
Katrin Cartlidge had originally been cast as Marianne, but she died shortly before filming was due to begin. Melissa Leo, who had previously auditioned for the role, was asked to return for another audition and subsequently secured the part.
Alejandro G. Iñárritu approached Naomi Watts about playing Christina whilst she was filming the well sequence for The Ring (2002). Watts feared that her scruffy wardrobe and hair would make her seem unprofessional, but it proved to be exactly the look Iñárritu was after.
The poem by the Venezuelan writer that Paul (Sean Penn) recites whilst at the restaurant with Cristina (Naomi Watts) is 'La Tierra Giró para Acercarnos', written by Eugenio Montejo.
The film's cast features three Academy Award winners — Sean Penn, Benicio Del Toro and Melissa Leo — and one Academy Award nominee, Naomi Watts.
The tale in Cruising Paradise that the pictures point to is titled "The Hero is in His Kitchen". It centres on a payphone conversation between two men — one in Tucson, Arizona, the other in Los Angeles, California. One of the closing lines on the story's final page (the page the pictures indicate) reads: "Don't buckle under the weight of a heavy heart!"
When the cast eventually received the final draft of the screenplay (they had all agreed to take part without having seen a script), each actor was handed a separate script for their own character, so they were not really privy to what was happening with the other characters.
The film's concept occurred to Guillermo Arriaga in a dream.
When Charlotte Gainsbourg's father, Serge Gainsbourg, was in hospital, he is reported to have stashed cigarette butts inside medicine bottles. In the film, Charlotte's husband, Sean Penn, likewise hides his cigarettes in a medicine bottle kept in the bathroom of their home.
One of two films released in 2003 that featured both Sean Penn and Kevin Chapman. They also co-starred in Mystic River (2003).
Sean Penn and Naomi Watts subsequently appeared in Fair Game (2010).
In the original draft of the screenplay, Cristina's surname was Beck rather than Peck.
Sean Penn and Denis O'Hare both feature in Milk (2008).
Lisa Sanchez made her debut.
This marked Carly Nahon's first screen appearance.
This film brings Sean Penn and Benicio Del Toro together again following 'The Pledge'; Penn takes the helm as director, and Del Toro appears amongst an ensemble cast that also features Mickey Rourke, Robin Wright, Vanessa Redgrave, Aaron Eckhart, Jack Nicholson, Patricia Clarkson and Helen Mirren.
