Joaquin Phoenix ad-libbed his scream of "Am I not merciful?" Connie Nielsen wasn't expecting it, and her frightened reaction was genuine.
Oliver Reed died three weeks before principal photography ended. Because Proximo was considered a key character, a clause in the movie's insurance contract would have allowed the filmmakers to re-shoot all of Reed's scenes with another actor at the insurer's expense, about $25 million. However, most of the actors and crew were exhausted from the punishing schedule, and Sir Ridley Scott did not want to cut Reed from the movie. The script was rewritten, and a body double and CGI were used to give Reed's character a plausible resolution.
Maximus' description of his home--specifically, how the kitchen is arranged and smells in the morning and at night--was ad-libbed. It's a description of Russell Crowe's own home in Australia.
Although Commodus was initially favored by the Roman people, he lost that status through dramatic acts of megalomania. He is often considered the initiator of the fall of Rome. During his reign, he incorporated his name into many common terms, such as the terms for money and the people. Eventually, the citizens and the Senate had enough of his rule: he was poisoned and, when he vomited out the poison, he was strangled. Afterward, the Senate returned the language to what it had been before Commodus, and the many statues of himself he'd put up were taken down.
Brian Blessed: A Colosseum spectator during the games. Blessed is best known for playing the Roman emperor Augustus in the acclaimed series I, Claudius (1976). Augustus was the grandfather of emperor Claudius, played by Sir Derek Jacobi, who portrayed Senator Gracchus in this movie.