When Rocky is training for the fight, he is sparring with a smaller quicker fighter. The sparring partner is played by real life Champion Roberto Durán.
After the bell rings, signaling the end of the second round, Sylvester Stallone and Carl Weathers are seen pushing, shoving, taunting, and ultimately being pulled apart by their respective cornermen. They continue to taunt each other before returning to their corners. Stallone revealed later they were actually angry with each other and were not acting at that point, several blows that were supposed to miss him landed and the carefully choreographed fight, which they spent months meticulously planning out, went off-track during that scene, but he liked the reaction the scene produced. He decided to leave their momentary breaking of character in and the viewing audience never realized the two actors were in reality quite livid with each other.
Sylvester Stallone himself wrote the paperback novelization for this movie. The novel is mostly in first person, from Rocky's point of view, written in the same choppy English in which Rocky speaks. Scenes in which Rocky is not present (such as Apollo Creed consulting his associates, or Paulie alone with Adrian) are in standard third-person, in proper English.
800 local schoolchildren were used as extras for the scene depicting Rocky's run through Philadelphia.
During his preparation for the film, Sylvester Stallone was bench-pressing 220 pounds, when the weight fell and tore his right pectoral muscle. This was shortly before the fight scene was to be filmed, and ultimately, the scene was shot with Stallone still badly injured.
Frank Stallone: Sylvester Stallone's brother briefly reprises his role as the Streetcorner singer from Rocky (1976).