Reese Witherspoon with Joaquin Phoenix in Walk the Line

Reese Witherspoon is the actress most likely to inherit the crown of Meryl Streep. She has the savvy, the work ethic and now, thanks to her performance as the singer June Carter Cash in Walk the Line, the biopic of Johnny Cash, the versatility, too.

Walk the Line tells the story leading up to the Cashes? devoted 35-year marriage ? the rocky years of failed early marriages and amphetamine addiction and transcendent music. It is Witherspoon?s first properly grown-up role, a feisty pained foray into tangled love and adult sexuality that opens up a whole new vista for her away from the frothy comedies such as Legally Blonde that made her a star.

Normally, perky sweethearts wait until their looks have weathered a touch before making the switch to knockdown premarital drama. But Witherspoon has done it at 29, overcoming the setback of absurdly fresh-faced looks. She grew up in Nashville, where the Cashes were a celebrated fixture. She once played June Carter?s mother in a school play about the singing Carter family, but that was no preparation for the amount of work required to play June herself.

?It was six months of learning how to sing and play the autoharp, five or six hours a day. Six months of studying videotapes of her in concert. I must have heard her comedy routines 700 million times. She had these little June-isms, telling the same story over and over again at each concert.?

The film?s director and co-writer James Mangold, best known for directing Copland and Girl Interrupted, was adamant that the actors would sing and pre-record their own parts. The two leads ? Joaquin Phoenix plays Cash ? were put through a ?rock ?n? roll boot camp? with a string of voice coaches to prepare them. Phoenix lowered his voice an octave to capture Cash?s profound bass rumble, while Witherspoon worked on June?s lilting inflections.

?When we started we sounded bad. We were terrified someone would get our first recordings and put them on the internet. The longer we worked on it the better we got. The coaches got us to sing from our diaphragm and not our head, but it took six months to get to a place where I felt comfortable enough to record.?

The film?s music producer T-Bone Burnett, who did the score for O Brother, Where Art Thou?, was brought in for the final stages. ?T-Bone was amazing,? Witherspoon says. ?He said: ?Sing it like you?re singing to your children?.?

?We had so much time together,? says Witherspoon about Phoenix. ?He?d get sick of me. I?d get sick of him. He?d make me stand further away because my singing was too loud. Then we?d make up and be friends. Some days, I was begging to get out of the part, I was so terrified of it. But Joaquin kept me going. By the time we started filming, we were a real team.?

?Our relationship was imperative,? she continues. ?We had to have a closeness. I don?t have any experiences in my career where I?ve needed another?s performance so badly, where it dictated whether I would be good or bad in a scene. I had to really find the beauty, not just in John, but in Joaquin.?

That beauty, it has to be said, is not immediately apparent. Phoenix is the rumpled bed-head type, tender but uneasy in his own skin and intensely reclusive about his work.

Witherspoon is the wholesome vegetarian, happily into her seventh year of marriage to actor Ryan Philippe and busy running her own production company Type A. When not working, she gets up with the kids and cooks their meals. When not working, Phoenix starts the day with a cigarette and then spends two hours looking at his e-mails.

Witherspoon didn?t get to meet June Carter Cash before she died in 2003, but she met the Cashes? children at June?s funeral and visited the family home in Nashville. ?People said June had two different personalities: onstage she was bubbly and funny. Offstage, she was very practical and tough.? For all her own steel magnolia poise, when it came to filming Witherspoon couldn?t contain her nerves. ?I lost my voice I was so nervous. I?m great with a camera, not singing in front of 3,000 extras.?

Come March 5, the Academy members may beg to differ.