There were many injuries acquired on the set. Both 'Josh Lucas' and Dermot Mulroney broke ribs during fight scenes between each other, and Jamie Bell stepped on a nail and had to walk on crutches for days.
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)".
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.
David Gordon Green originally wanted to title the film "The Narrow Pilgrim," focusing on Tim in his pilgrim costume at Thanksgiving. The original story was going to put Tim more in the focus, along with his eating disorder. He is thin and ratty, due to his pica condition, and in a pilgrim costume: "The Narrow Pilgrim." However, he quickly began to change it due it him discovering that it would not be a very marketable title.