Writer Anthony Tambakis portrays a fight official in the film, and Gavin O'Connor plays the fight promoter. O'Connor agreed to play J.J. Riley only after his friend and co-writer Tambakis agreed to make a cameo himself.
Instead of going to his hotel after his flight from England, Tom Hardy appeared at director Gavin O'Connor's door at midnight the evening before his audition. The pair ended up living together for five days.
Anthony Tambakis and Gavin O'Connor selected The National song "About Today" 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.
The role of the promoter, played by director Gavin O'Connor, was originally written for TapouT founder 'Charles 'Mask' Lewis'. Lewis was killed by a drunk driver just before shooting began. O'Connor spoke at Lewis' memorial service on April 14, 2009, six days before principal photography began. The film is dedicated to him.
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 universal critical praise.
Frank Grillo based his character on famed MMA trainer Greg Jackson. Grillo and Joel Edgerton trained and lived with Jackson at his New Mexico gym during pre-production. All of Grillo's fight scene dialogue while "cornering" Edgerton was suggested by Jackson.
The footage shown of Tommy Riordan wrestling in high school is actually high school footage of Pennsylvania high school standout, and later NCAA Division I champion and US Olympic team member Cary Kolat. Kolat was also unbeaten during his high school career as it is suggested Tommy was.
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.
The original script location for the Conlon family was Long Beach, CA. It was moved to Pittsburgh due to Pennsylvania tax breaks. The scenes set at Boardwalk Hall in Atlantic City were originally written for the MGM Grand in Las Vegas. Gavin O'Connor moved the action to New Jersey to fit with the gritty east coast aesthetic of the film.
Olympic gold medalist Kurt Angle played the Russian fighter "Koba," whose nickname was taken from the moniker given to 'Josef Stalin'. In the original script, "Koba" was named "King Kong," but it was changed for legal reasons. The brother of Gavin O'Connor's assistant, Samantha Ellison, suggested the name "Koba."
One of the commentator's references Kurt Angle's character, Koba, as being a World Sambo Champion. Sambo is a martial art known for leg locks - Angle employed an ankle lock as a signature move during his wrestling career.
The character "Koba" seems to loosely resemble real life Mixed Martial Artist Fedor Emelianenko, who was the #1 ranked Heavyweight in Mixed Martial Arts for over 7 years and considered unbeatable. He also was Russian and a Sambo Champion multiple times over. Plus there is even a physical resemblance, as Fedor was white, bald, and about Koba's size. However, the name 'Koba' was a moniker for Josef Stalin. Emelianenko's reputation as a professional athlete as well as his character are highly positive, and his physique is pudgier than that of Angle's, so the similarity is only in their professional achievements.
Joel Edgerton's character may be loosely based on the life of real-life UFC fighter Rich "Ace" Franklin. Like Edgerton's character, Franklin was also a high school teacher who took part in cage-fights to make extra money.