Joaquin Phoenix and Reese Witherspoon performed all of the songs themselves, without being dubbed. They also learned to play their instruments (guitar and auto-harp, respectively) from scratch.
It took 4 years for the producers to secure the rights to the story from James Keach, who is a friend of Johnny Cash and his family. After Keach agreed, it took another 4 years to get the film made.
Johnny gets fan mail from a Folsom Prison inmate named Glen Sherley. In real life, Sherley was an inmate at Folsom when Johnny recorded "At Folsom Prison." He also wrote "Greystone Chapel," which Johnny recorded during the show.
According to director James Mangold, when Joaquin Phoenix was learning how to sing and play guitar like Johnny Cash, his voice was too high and the band learned how to play Cash's songs in a higher key. Just before filming started, Joaquin's voice dropped closer to John's level, and the band had to re-learn the songs in their original key.
When Johnny Cash wakes up on the tour bus, just after the Folsom Prison performance, he walks past guitarist Luther Perkins, who is passed-out with a lit cigarette in his mouth, and puts the cigarette out. Perkins died a few months after the 'At Folsom Prison' recording/performance. He fell asleep in his Tennessee home with a lit cigarette in his mouth, and died from injuries sustained in the resulting fire.
Johnny Cash and June Carter Cash's actual home, in Hendersonville TN, appears in the movie. It burned to the ground on April 10, 2007 while under renovation by its new owner, Barry Gibb, of the Bee Gees.
When Johnny Cash first approaches the recording studio, a pair of young men are rhythmically polishing shoes; an homage to Johnny Cash's song "Get Rhythm," in which he sings about a shoeshine boy on the corner of the street.
After visiting many of Johnny Cash's old homes, production designer David J. Bomba created 90 different sets for the film and tried to underline the contrast between Cash's two lives, one that was close to earth and nature in Arkansas and Tennessee, and the other set in the fast-moving world of rock music.
Joaquin Phoenix wears 56 different costumes. All were designed by Arianne Phillips following meticulous research within the Cash family's archives and fans' private collections.
Cinematographer Phedon Papamichael shot the concert scenes mainly with hand-held Super 35 cameras as he didn't want to give a false sense of glamor to the performances.
Towards the end of the movie Johnny tells his Dad to tell the girls about the flood. This is a reference to a real incident in Johnny's childhood when the family farm flooded that he wrote and sang about in his famous song 'Five Feet High and Rising'.
In the 1956 Sun Records recording of "I Walk the Line," Johnny Cash flubs the final low note ("because you're MINE"). Joaquin Phoenix flubs the same note, in the same manner, in the film, as he sings "I Walk the Line."
Johnny Cash really proposed to June Carter onstage. It happened in February 1968, at the London Ice House, a hockey arena in London, Ontario, in the middle of a performance of "Jackson." She accepted, and they married a week later.