Most people think this film was a Sylvester Stallone project, but according to Robert De Niro he got the script first and quickly came to the conclusion that Stallone would be the best choice to play his opponent. Stallone himself was actually quite reluctant to do the film at first, as he did not want to parody Rocky (1976) and De Niro had to talk him into it. Stallone claimed that he had a harder than usual time training for the role since he was taking a break from action roles and had stopped working out as much as he normally does.
Alternate endings were filmed to avoid any spoilers or leaks.
Sally Rose is played by Kim Basinger in the 2010s scenes, and her daughter Ireland Baldwin in the 1980s scenes.
In a scene, while training, "Razor" (Sylvester Stallone) is about to punch hanging raw meat in a meat locker and is told not to by "Lightning" (Alan Arkin) because it is unsanitary. This is a nod to the scene in Rocky (1976) where he does break the ribs, and obviously doesn't care if it is unsanitary.