Joaquin Phoenix ad-libbed his scream of "Am I not merciful?" Connie Nielsen wasn't expecting it, and her frightened reaction was genuine.
Russell Crowe later admitted that he initially felt unworthy of all the praise and the Oscar for Best Actor that he got for this movie. After seeing the completed movie, he felt it was a "director's film", and that the Oscar should have gone to director Sir Ridley Scott instead.
The wounds on Russell Crowe's face after the opening battle scene are real, caused when his horse startled and backed him into tree branches. The stitches in his cheek are clearly visible when he is telling Commodus he intends to return home.
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.
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.