During the shooting the scene where Deel drives Chris on the dirt road away from the farm, a police chopper was continually circling the area due to a dead body being found around the area. The cast and crew never saw the actual dead body, however.
Devon Alan's (Tim) monologue about "Chiggars" was not in the script and was totally improvised by the 11 year old.
During their fight scene, Josh Lucas actually blacked out while being choked by Dermot Mulroney.
The United Artists logo seen in the opening credits of the film was from the company's archives. All the films released under United Artists at the time of Undertow's release had a different logo. As a child, director Green always saw that logo and was unsettled by it, and for that reason, he used it in the film.
Josh Lucas had to wear brown-colored contacts in the film to disguise his piercingly blue eyes. Director Green said that it was to mask what made girls fall for him in his role in "Sweet Home Alabama (2002)".