Phoenix, who sang his own numbers in the movie (and is heavily favored to receive a Best Actor Oscar nomination when they are announced Jan. 31), played several songs at the prison's Greystone Chapel, including Cash's Folsom Prison Blues, the Associated Press reports.
Referring to his on-screen costar Reese Witherspoon, who was a Folsom no-show, Phoenix said: "I know you guys would probably rather see Reese."
Outfitted in Cash's trademark black, a self-critical Phoenix, 31, apologized for his "rusty" performance, explaining that he hasn't played since he wrapped the movie. "I don't know if you've noticed, but I've messed up like 40 times," he said. "I'm all over the place."
One inmate asked Phoenix if he had learned to play guitar for the movie. Replied the actor: "John wasn't (Jimi) Hendrix. It was real simple ... we rehearsed a lot."