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.
June Carter Cash died before production began on the film, so Reese Witherspoon's research included looking through Carter's closet for inspiration.
Sony, Universal, Focus Features, Paramount, Columbia Pictures, and Warner Bros. all passed on the project.
It took four 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 four years to get the film made.
One of the fan letters Johnny receives is from an inmate at Folsom Prison named Glen Sherley. Glen Sherley was an inmate at Folsom when Johnny recorded "At Folsom Prison" and also wrote the song "Greystone Chapel" that Johnny recorded during that show.
According to director James Mangold, when Joaquin Phoenix was learning how to sing and play guitar like Johnny Cash in the months following the start of filming, his voice was too high and the band had to learn how to play Cash's songs in a higher key. But then just before they started filming, Joaquin's voice dropped closer to John's level and the band had to re-learn the songs in their original key.
At 5' 8" tall, Joaquin Phoenix is six inches shorter than the 6'2" Johnny Cash.
When Cash collapses from a drug overdose he says, "Fortunately, I keep my feathers numbered for just such an emergency." That is a line from a Foghorn Leghorn cartoon after all his feathers are blown off by a stick of dynamite and it is referred to earlier in the movie when Johnny and Jack are walking together.
The scene where Johnny Cash pulls the sink off the wall was not scripted. Joaquin Phoenix actually pulled it off the wall.
When Johnny Cash calls his wife during his first show, a sign on the wall says: RING in the case OF FIRE.
When Johnny Cash wakes up on the tour bus, just after the Folsom Prison performance, he walks past a passed-out Luther Perkins (his guitarist) who has a lit cigarette in his mouth, and he puts out Luther's cigarette. Luther Perkins died 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.
Elvis Presley's band members in the film were played by The Dempseys.
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 reconstruction by its new owner, Barry Gibb, of Bee Gees fame.
In the scene where we see the The Tennessee Three for the first time ("Rock and Roll Ruby"), there is a sign on the wall behind the audience that says "CLASS OF 1955". This may be a reference to the group set up by Johnny Cash, Carl Perkins, Jerry Lee Lewis and Roy Orbison, called "Class of '55".
In the scene where 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.
Waylon Payne originally auditioned for the role of Waylon Jennings (in real life, Jennings was Payne's godfather). However, the director was so impressed by Payne's audition, as well as his fire and spirit, that he cast Payne in the much bigger role of Jerry Lee Lewis. Interestingly enough, the actor who got the role of Waylon Jennings was Waylon's own son, Shooter Jennings.
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 throughout the course of the film which were all 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.
The film was screened for the inmates of Folsom Prison, 38 years after Johnny Cash's landmark performance.
Shelby Lynne, who portrays Johnny Cash's mother Carrie grew up listening to Cash's music and composed the song Johnny Meet June on the day of his death.
Joaquin Phoenix was hospitalized after completing the filming of this movie, in order to deal with the similarities between his life and Johnny Cash's
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."