Joaquin Phoenix ad-libbed his scream of "Am I not merciful?" Connie Nielsen wasn't expecting it, and her frightened reaction was genuine.

Five tigers were brought in for the scene in the arena where Maximus fights Tigris the Gaul. A veterinarian armed with tranquilizer darts was present the entire time. For safety's sake, Russell Crowe was kept at least fifteen feet away from the tigers.

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.