Joel Edgerton tore his MCL in the cage during production, halting fight scenes for six weeks. Tom Hardy suffered a broken toe, broken ribs, and a broken finger.
The role of Paddy was written for Nick Nolte by Anthony Tambakis and Gavin O'Connor, who are neighbors with the actor in Malibu. The studio was resistant to casting Nolte, but the writers held firm and Nolte's portrayal has won him global critical praise.
When the announcer is quoting Koba's credentials, he states that he is an Olympic Gold medalist in wrestling. Kurt Angle, the actor playing Koba, won a gold medal in the 1996 Olympics in the 220lbs/100 kg weight class for freestyle wrestling. He was also a world champion in the same style and weight class in 1995.
On the first day of shooting, the crew gave Nick Nolte a standing ovation after the first take of a scene shot at a local diner. The scene was later cut but appears as a DVD extra.
Anthony Tambakis and Gavin O'Connor selected the song "About Today" by The National to close the movie, before writing the final scene. The scene was written with the song playing on a continual loop at O'Connor's house while the writers worked.