Musical idol worship isn’t just for love-struck teenage girls on the verge of passing out or lifer criminals identifying with a guy singing about firing .45s on a 45.

Performers themselves can fall prey just as easily to the rock ‘n’ roll mystique. That idea makes Walk the Line a fascinating love story and biography about Johnny Cash, June Carter, the music they made and their tumultuous decade-plus courtship.

By nearly all accounts, Cash and Carter’s marriage was one of puppy-dog affection. Judging from the movie, it’s because they got a lifetime’s worth of barking out of the way beforehand. How they achieve wedded bliss is not the typically uplifting story of people saving people.

These hardheads must cut through romanticized views each has of the other to get at what’s real — that they’re two people with similar self-esteem imperfections born from different circumstances. Their getting together is both true love found and defenses worn down, and it feels exactly right.

Their great music, expertly re-created by Joaquin Phoenix (as Cash) and Reese Witherspoon (as Carter), also factors into this unconventional romance.

Alone, some of Cash and Carter’s songs feel like impassioned pleas to the other. Together onstage, they make love, bicker and reconcile three inches apart over a three-minute song. Phoenix and Witherspoon’s body language is brilliant — lustful, excited and confused all at once. Their minds and their words are having a torrid affair, even if their bodies aren’t.

This connection begins before they even meet, with a late-night radio broadcast during which young Johnny becomes enamored with June’s voice. His own musical dreams are ridiculed by a totalitarian dad (Robert Patrick) and too strongly influenced by the death of his brother Jack, who dreamed of being a preacher before a workshop accident.

After an Air Force stint and starting a family with wife Vivian (Ginnifer Goodwin), Johnny plays his groaner gospel music to Sun Records’ Sam Phillips (Dallas Roberts).

Roberts delivers the only bit of music-mythology speechifying by telling Johnny he needs to play “somethin’ real, somethin’ different, somethin’ you felt.” But the payoff is a camera that holds on Phoenix’s slowly building confidence and rendition of “Folsom Prison Blues” that gives birth not only Johnny’s music career, but the true start to his life.

While on tour, Johnny meets June and learns her shadow of self-loathing stand almost as tall in a family filled with talent. Thought of as a lesser singer than her sister Anita, June has settled on a spunky, “good Christian” comedienne persona. Johnny and June’s thoroughly enthralling back-and-forth involves drug abuse, fan expectations, domestic discord and shifting music-industry tastes.

Neither Phoenix nor Witherspoon provides an external mirror-image performance, but they can provide utter immersion into these roles.

“Friend John, guy Cash” is a good in-the-dialogue summation of Phoenix’s performance, with a drawl that’s alternately charming and predatory and expressions of both happiness and drug-gripped mania. In her best performance since Election, Witherspoon’s chubby-cheeked chipmunk look is gone, replaced by a gaunt face and a now tough-and-determined Southern drawl. Her stubbornness and devotion feel born of concern that her admiration for Johnny isn’t the right thing.

Walk the Line pays homage to these musical giants precisely by portraying them as anything but. It’s only peripherally about the singles and sales — it’s about how the songs, in part, were a long, complicated search for each to find the man in the Man in Black and the June in June Carter.