Antoine Fuqua had no money to pay James Horner to compose the film, due to the film's short budget. However, Horner did not care as he loved the idea of the film, and he decided to do it for free.
Eminem was originally scouted for the role, which he accepted and started filming. The film had to be put on hold as Eminem wanted to focus on his music career, so the role was then given to Jake Gyllenhaal.
Some prints of the film, at the end of the credits, have "In Memory of Our Friend James Horner," who died in a plane crash days before the film opened for release.
For five months, Jake Gyllenhaal undertook a daily routine of 2,000 sit ups, 200 push ups and squats, 100 pull ups and a six mile run for this role.
The song that plays at the end of this film, "Wise Man" by Frank Ocean, was originally written to be in Django sin cadenas (2012). Quentin Tarantino wasn't able to find a scene to place the song in, so it was eventually scrapped from the final cut.