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 the Man washes the 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. The boy's crying afterward was Smit-McPhee actually crying, not acting.
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 the motion picture. 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.
The cause of the global catastrophe is never specified. While both the book and the film strongly hint at two explanations (a worldwide nuclear war or a massive strike on the Earth by space-born objects), the filmmakers did not want to make the film about the characters' blame or lack thereof for the situation, and keep the focus on how The Man and The Boy would try to survive in the post-apocalyptic settings.
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.
An apparent CGI aerial shot, which also appears in the trailer, is a digital recreation of actual 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.
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 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".