To live the role, Viggo Mortensen would sleep in his clothes and deliberately starve himself. At one point, he was thrown out of a shop in Pittsburgh, because they thought he was a homeless man.
During a preview Q&A screening in London, John Hillcoat revealed that Kodi Smit-McPhee won the role of "The Boy", partly due to an audition tape sent in by Kodi's father that showed them re-enacting the scene where the father shows the boy how to kill himself, by placing a pistol in his mouth.
The scene where Man washes Boy's hair in the stream was shot three times. During that scene, the weather was very cold, so John Hillcoat promised Kodi Smit-McPhee that it would be done in only two takes. However, during the second take, the sun came out and ruined the shot, requiring a third take. Boy's crying afterward was Smit-McPhee actually crying, not acting.
Viggo Mortensen and Kodi Smit-McPhee reported bonding by eating crickets to help them get into character.
Director John Hillcoat filmed the soft drink vending machine scene with Man and Boy several times, each with a different brand beverage, out of concern that Coca-Cola executives would not want their product to appear in this movie. A telephone call, from Viggo Mortensen to the President of Coca-Cola, secured permission for a can of Coca-Cola to appear, consistent with the source novel.
Viggo Mortensen nearly turned down the role of Man, because he had planned a break from film work. After completing his work as Man, Mortensen took roughly two years off from acting.
Kodi Smit-McPhee won his part over hundreds of boys due to a strong audition and his resemblance to Charlize Theron, who had been cast as Woman.
One of the cannibals on the back of the truck was played by Kodi Smit-McPhee's real-life father.
Nobody in the film is credited with a specific name. The only character who does refer to himself by name is Old Man, who calls himself Ely.
John Hillcoat did shoot the controversial scene from the book involving a baby on a spit being roasted over a campfire, but ultimately decided to cut it, because he felt it was simply too much.
One of the reasons Joe Penhall landed the job writing the screenplay, was because he felt there was no need to change the novel's dialogue.
Old Man's line about having a son was not in the script, and was ad-libbed by Robert Duvall, after they were all tired from many takes.
For the birthing scene, John Hillcoat originally planned to record the screaming of women actually giving birth, and edit it over the scene. Charlize Theron objected to this, and filmed the scene herself. The screaming you hear in the film, is actually her.
Viggo Mortensen claimed that he never kept track of how much weight he lost for the role, as it was a gradual weight loss as the result of less eating, but he estimates it was approximately thirty pounds.
Whenever it was a sunny day, the visual effects technicians had to use CGI to make it look cloudy, because John Hillcoat wanted to maintain a desolate atmosphere.
Kodi Smit-McPhee's father read the whole book to Smit-McPhee right before he auditioned for the role of the boy.
Boy's messy, spiral crayon drawings are reminiscent of a famous David "Chim" Seymour photograph from 1948. In it, Tereska, a psychologically disturbed child Holocaust survivor, drew similar circular, messy lines to represent "home".
The cause of the global catastrophe is never specified. While the book and the film strongly hint at two explanations (a worldwide nuclear war or a massive strike on the Earth by spaceborne objects), the filmmakers did not want to make the film about the characters' blame, or lack thereof, for the situation, and kept the focus on how Man and Boy would try to survive in the post-apocalyptic settings.
Director John Hillcoat first read the novel and fell in love with it, before it was published.
The shooting schedule involved nine hours of shooting during each day. Kodi Smit-McPhee was unavailable for three of those hours, as he was being schooled.
The screenplay for this film was featured in the 2007 Blacklist; a list of the "most liked" unmade scripts of the year.
A CGI aerial shot, which also appears in the trailer, is a digital re-creation of destruction by Hurricane Katrina to Empire, Plaquemines Parish, Louisiana. The shot shows large, twin boats on a highway in front of a bridge over the Empire Lock on Louisiana State Highway 23. In the movie rendering, a large city skyline appears on the horizon, where, in actuality, there would be only the rural peninsula of Plaquemines Parish.
Despite the prominent billing they receive, Robert Duvall (Old Man) and Guy Pearce (Veteran) get very little screentime.
John Hillcoat originally wanted to shoot this film in chronological order, but the logistics of filming at numerous locations in several different states made this an impossibility.
Half the crew members for the shoot in New Orleans, Louisiana were survivors of Hurricane Katrina.
The U.S. release date was postponed from November 26, 2008, to October 16, 2009, to allow for additional post-production work. The 2009 release date was further delayed to November 25, 2009, to position the picture for Academy Awards competition.
The scene outside of the tunnel was filmed at the Rays Hill Tunnel in Bedford County, PA; part of a 13-mile stretch of what is called the Abandoned Pennsylvania Turnpike. The tunnel was originally built in the 1880s to serve as a railroad tunnel but was never used. In the 1930s, the tunnel was repurposed to become part of the new Pennsylvania Turnpike system. In the 1960s, a 13-mile stretch of the turnpike was re-routed to relieve traffic congestion and this original part, which included two tunnels, was simply abandoned.
The name of Old Man is given as Ely. This is also the name of the town in which Guy Pearce (Veteran) was born, in Cambridgeshire, England, although the pronunciation is different.