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 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.
During a preview Q&A screening in London, director 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.
Director 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 said scene from the final finished film because he felt it was simply too much.
For the birthing scene, director 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.
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 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.
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.
In the movie the man talks about the pistol he is using as shooting faster than the speed of sound. While in the movie they used a Smith &Westen 1917, which shoots a .45 CAP round. The muzzle velocity of this gun is 975 fps while the speed of sound at sea level is 1100 fps.