Reese Witherspoon is gushing. We are sitting in the Hotel Bel Air, where we have met to have high tea (English Breakfast for her, with lots of honey) and—more important—to talk about Walk the Line, the movie in which she and Joaquin Phoenix star as Johnny Cash and June Carter Cash. But she really wants to talk about Dolly Parton, whom she has just met at Vogue's photo shoot in Nashville, where, she says, she cried—a lot. "Dolly was amazing—so sweet. I just geeked out on her, and she let me. I told her all this stuff—I told her I loved 'Coat of Many Colors' and that I sing that one to Ava at night and that I have a storybook that goes along with it. And then I got kind of quiet because Joaquin talked to her the whole time and it made me so nervous and I said, 'Oh, gosh, will you please leave her alone,' and she said, 'Now, honey, I'm married, but I'm not dead.' I was like 'Yes! She did a Dollyism!' Then she played 'Nine to Five' on her acrylic nails. It was the greatest thing ever. She sang a bunch of Johnny's songs, and that's when I started to lose it. She sang 'I Still Miss Him,' and I just burst out crying, and then when she left I really had a nervous breakdown." Not surprisingly, someone at the shoot asked her if she was upset, and she told her—repeating it to me now in a mock wail—"No, I just don't know how I got to be so lucky. I got to play June Carter and I got to meet Dolly Parton."

Neither would likely rank as high among the dreams of most movie stars, but Reese Witherspoon grew up in Nashville, "Music City USA," where, as she points out, you can't avoid country music. Her mother, who holds a Ph.D. in pediatric nursing, loves pop country, and Reese's favorite song when she was six was the Dolly Parton/Kenny Rogers duet "Islands in the Stream." Even though she went to elementary school in Belle Meade, a wealthy old guard city within the city, her fourth-grade play, Nashville in a Nutshell, featured "performances" by Loretta Lynn, Barbara Mandrell, and June Carter's legendary mother, Maybelle Carter, who was played by none other than the nine-year-old Reese, who asks me now, "How weird is that?"

Well, actually no weirder than, say, the idea of Elle Woods, Witherspoon's fashion-obsessed, perkier-than-thou Legally Blonde character, suddenly morphing into the soulful, down-to-earth—and brunette—Carter, whose stormy and redemptive relationship with Johnny Cash is the subject of Walk the Line. Witherspoon is not Woods, of course; she probably has a lot more in common with Carter—the Tennessee background, the religious faith, a no-nonsense nature that does not suffer fools. But to much of the moviegoing public, she is the lovable star of hugely successful romantic comedies—one not easily imagined as a deeply loving, thrice-divorced woman capable of pulling a man back from the brink.

But, she tells me, sounding more like Woods again, "I'm always up for a challenge."

And it definitely sounds like one. There was the intense six-month prep during which she had to learn to play the autoharp and sing. There were the grueling nights of shooting in buggy, humid Memphis, where, she says, she endured "two infestations of chiggers and more mosquito bites than I've ever had in my life. I'd go from 'I can't marry you, you're a drug addict' to 'Can we just cut? I'm being eaten alive here.' " And then there was the material itself. Witherspoon, a self-described homebody and mother of two who is married to actor Ryan Phillippe, says she could never, in her own life, put up with Cash's brand of self-destructive behavior. "John wasn't exactly a real chipper guy. But there's something very beautiful and poetic about someone who's such an amazing songwriter and really spiritual and sensitive. I mean, I completely see the appeal."

So do I, especially as Cash is played by the smoldering Phoenix, in whose presence I have a hard time remaining entirely in control. "John saw something in June that no one else did," says the actor, whom I met a few days later sitting under the eaves of the dining terrace at Los Angeles's Chateau Marmont hotel. Carter had been the singing comedienne of the Carter family, the hardworking clown, but Cash encouraged her to come into her own as a singer—and as a woman. As Witherspoon describes it, "He opened up her sexuality. He freed her from her persona and just completely broke open the idea of who she was."

The irony is that the role completely breaks open the idea of who Witherspoon herself is. It's the first time she plays a truly grown-up, sexual woman, for one thing—in Sweet Home Alabama she had not one but two leading men, and she shared maybe a handful of kisses between them. Her own persona is funny, upbeat movie star, to be sure, but she can also come across as an almost irritatingly good girl who is maybe just a little tightly wound. The reality is a lot more complicated. And the dichotomy between the two is exactly what made director James Mangold think his star could not just pull off the role but imbue it with surprisingly moving depth.

While her Nashville roots were appealing to the director—and they have the added bonus of saving the rest of us from the usual over-the-top Hollywood Southern drawl—Mangold says he was drawn to other similarities between his star, who made her first movie at fourteen, and his character, who'd been onstage since very early childhood: "I really began to feel that there were two Junes, the onstage June and the backstage June." Likewise, he says, "I had known Reese, and I was always impressed with the confident comedic persona she had in those movies, but I also knew the Reese who was a businesswoman and a mother who is really, really smart, with this extraordinary honesty. It wasn't like there were two people, but there was a persona created for her profession and then there was a person. I thought this role was a way to bring the two together. I mean, cycle through the actors of her generation and tell me who could put that shine on onstage and as soon as the curtain comes down be so real and so grounded."

Witherspoon, who attended Broadway summer camps as a kid to learn to sing and dance and act as "a hobby"—she enrolled in Stanford with the intention of becoming a doctor—also shares Carter's innate showbiz discipline. "Johnny Cash and the musicians at Sun Records were like the young guys in the Actors Studio," says Mangold. "What mattered was if it was alive and real, and if it went bad, so what? June came from a different tradition. She was raised onstage, and that show-must-go-on mentality was hardwired into her DNA. Reese really gets that professionalism." Indeed, in a scene in which a frustrated Carter walks into rehearsal to find Johnny and the boys drinking beer rather than practicing, she pelts them with the empty bottles Witherspoon says she had no problem letting fly. In another, after he had let her down again, Cash says to Carter, "Tell me you don't love me," and Witherspoon's character shoots back that she does not. "After the first two takes, Jim comes over and says, 'Reese, it really looks like you don't love him,' " she tells me, laughing hard. "And I thought, Hmm, maybe I should rethink my response. But he wasn't particularly lovable at that point."

Her discipline served her well in preparing for the role, which she did by studying performance tapes, reading the couple's biographies and autobiographies, and talking to their kids, since both died, within months of each other, before she had a chance to meet them. Phoenix, on the other hand, had spent time with them in a coincidence even more extraordinary than his costar's grade-school casting. Three months before Mangold asked him to do the film, he got a very different call, from a friend of the Cashes'. They were in L.A.; Johnny had loved him in Gladiator; would he like to come to dinner? He went, of course, and after dinner, Cash, who was weak with a panoply of ailments, sat down in the living room to play. "John started strumming a bit and humming along, and at one point he looked up kind of apologetically and said, 'I'm just waiting for June to get my nerve up.' And then June comes out and they start singing and they're looking into each other's eyes, I swear. I'm cynical when it comes to duets because I think I've been polluted by seeing Sonny and Cher and Nick and Jessica—it just always seems like a bad performance. And here are these people, and it wasn't a performance—they were among friends—and their love for each other seemed so unique and uncontainable and like something I don't know if I'd really believe if somebody told me. I'd say it was a publicity stunt, but with them it was so true and so honest."

Re-creating that intimacy and authenticity was not least among the movie's challenges. "Playing characters with such palpable love for each other—it's a strange process," says Phoenix. "We didn't know each other, and suddenly we're supposed to be lovers for life. What is that? But Reese was my pillar from the beginning. I was just amazed by her work ethic, and it was nice because it was so scary and completely brand-new for both of us in many respects. She really put her heart and soul into it."

Rarely is the film—or their chemistry—more powerful than when they are onstage together, an astounding achievement given the fact that neither of them had ever played an instrument or sung professionally. "It was really, really hard. I was so darn nervous I had to have a bucket nearby in case I was going to lose my lunch before I got up on the stage," Witherspoon says. "And I tried to get out of it like six times. I said, 'Jim, can't you just get LeAnn Rimes to sing for me?' " But she does sing, beautifully, as does her costar, who credits Witherspoon with making it happen. "When it was decided that we would do the singing, Reese was the one who really got things in motion," says Phoenix. She found them vocal coaches, a guitar teacher, an autoharp teacher. "I didn't know what to do or how to approach it, and she was like, 'Well, at two o'clock, we're going here and at four-thirty, we're going here.' She had fewer songs than I did, and she could have easily done her own thing, but she cared about the film as a whole."

The latter is a sentiment echoed by many who have worked with the actress. Robert Harling, the screenwriter who rewrote much of Sweet Home Alabama, says that when he was brought in—in the middle of shooting—she called him from the set. "But it wasn't the usual stuff about 'me, my character, what are you going to put in my mouth?' It was about the story in its entirety. She wanted to protect the integrity of a film about the South." Harling, who wrote Steel Magnolias and is from Natchitoches, Louisiana, where Witherspoon made her first movie, The Man in the Moon, is currently working on a script for the actress called Sports Widow and reports that she has been remarkably helpful. "She is incredibly funny and a storyteller herself. She comes from a community of strong Southern women—we talk about our mamas a lot and what fodder they are for comedy and drama."

There is indeed plenty of fodder. She tells me her grandmother used to pick her up from elementary school every day in a big white Cadillac wearing driving gloves, and that her mother has herself become such a fanatical grandmother to six-year-old Ava and two-year-old Deacon, she introduces herself to everyone, even to adults, as "Grandma Betty." "My mother is just the most open, happy, laughing Southern lady. She hasn't met anybody she doesn't like. She thinks everybody is funny and everybody has a story to tell." Witherspoon shares with me the quintessentially Southern expressions of her father, a surgeon and ear-nose-and-throat specialist. "Dad always says someone is as crazy as two squirrels in a box—'She's a box of squirrels, baby.' " (He is also fond of "Make hay while the sun shines," something his daughter has clearly taken to heart.) She tells me that the fact that her husband has been cast as one of the leads in the upcoming Clint Eastwood movie Flags of Our Fathers has put her father over the moon, and she jokes that her family could well be part of a Jeff Foxworthy "You know you're a redneck if…" monologue. "Our working TV is on top of the broken TV—that's us." In Nashville, where we first meet, she shows me the driveway of the house where she grew up, which is full of old cars, including a white Mercedes ragtop Witherspoon drove in high school (the prestigious Harpeth Hall for girls), and says the family's new house, which she bought after National Enquirer began knocking on the door of the old one, is even more packed with automobiles. Her father and brother work on them every weekend—"It's like Monster Garage out there."

She is clearly at home here—to the point where she says she may buy another house in the country where her immediate family could live part-time. She says she has learned to cook chicken-fried steak and collard greens for Phillippe, "who loves the food here." And over lunch she attacks a fried-green-tomato salad (albeit one with no dressing) with gusto and drinks glass after glass of fruit tea, a locally revered concoction of iced tea with a mysterious mixture of juices she says her children are addicted to. She has spent the morning with her kids in the park, and I hear her making plans for the evening with an old high school friend "if I can find a baby-sitter." She says she loves that it's so easy to get around, that "you can do a million errands in an hour and still have the rest of the day to sweat." She says she wants her children to know their Southern roots and finds "value in going to church on Sunday and knowing your neighbors." In L.A., where she lives in the Hollywood Hills, "you can do the same things but somehow it feels different. I think part of it is the people and their attitude—in the South they are more friendly and more willing to be open." But there is also the fact, she says, that "L.A. is such a sprawling huge place, you feel so disconnected from different socioeconomic groups—it's more segregated than the South. Even if you did community service, you would never see those people again. At home, you'd see them in the grocery store."

In Nashville, folks approach her, she says, "But they're respectful. They only want to say hi and see the baby. Or they want to tell you how they know you or how they're related to you, which is funny—I seem to have some connection to pretty much everybody." In L.A., on the other hand, she has been famously hounded by the paparazzi. In September, a photographer was arrested after he allegedly shoved and hit employees at the California Adventure theme park who were escorting a group that included Witherspoon and her children, and she tells me that recently when she dropped her kids off at preschool there was a "creep" wearing camouflage hiding in the bushes. "First of all, Jerky, I see you. You're not tricky, and that outfit isn't doing a thing for you." She has a full-blown Southern accent going now—she means business to the point where she jokes that she'd like to make like an old nanny of mine I'd told her about who always packed a gun in her handbag.

The low-key life she tries to lead—it should be noted that almost all the paparazzi shots of her are in the grocery store or with her children—is in stark contrast to the lives of country-music singers, especially of the Carter-Cash generation, who played out their entire lives onstage, where, unbelievably, Carter finally accepted Cash's marriage proposal. "That's one of the things Joaquin and I talked about—why these people would do that—because we're very private people in our personal lives. I mean, what kind of woman stands up there and goes 'OK!' to an onstage proposal?" Phoenix says what finally made it work for them was that "at that point so much of John's life had been public. There was a connection with the audience. I think they felt a level of comfort."

Understandably, neither star shares it. Witherspoon is especially bewildered by the increasing plethora of magazines that exist almost solely to photograph and speculate on the romantic lives and marriages—including her own—of celebrities. "There are no words in those things—a monkey could make those magazines," she says just before covering her mouth. "That sounds really bad—did I just say that? But really, does anybody read anything anymore?"

Like her character June Carter—at one point in the film, Johnny looks in June's suitcase and says, "Girl, you got a library in there"—Witherspoon is a voracious reader. "You know, it took me a long time to get over being embarrassed about being from here. I think there's a built-in idea that you're not very cultured or educated, and I never wanted people to think I was stupid because I was from the South, but now I feel like it's such a huge part of who I am."

There is little danger these days of anybody thinking she is stupid. She plays a doctor completing her residency in Just Like Heaven, the romantic comedy that opened in September and costars Mark Ruffalo as the new tenant of her apartment whom she haunts after becoming the victim of a car accident. "When I go through the list of actors in her age range, there are very few about whom I could believably say, 'Yes, this woman is a doctor,' " says Mark Waters, the director of Mean Girls, who directed Witherspoon in Heaven. "The audience can believe that about Reese—it doesn't seem like a fake movie conceit. The defining characteristic of Reese is someone who's really intelligent." In Walk the Line, there are more than a couple of scenes in which Witherspoon wrote or adlibbed her lines, a development that delighted Mangold. "Both Joaquin and Reese were so inventive. If you've got the horses, let them run."

Right now, Witherspoon is concentrating on her film-production company and is thinking of developing a TV show set in the South, "something young but not silly." And she remains closely involved with her film projects. "Reese is brilliant in the business of building a character," says Harling. "Often what you get from an actor is 'I don't know, I just want to be happy, whatever.' But Reese is very clear about the journey she wants to take the audience on," and, he adds, about her own journey as well. "With each movie I see her setting the bar higher."

She has certainly cleared a big one in Walk the Line, which may come as a surprise to those who didn't know Witherspoon had it in her to feel the pain and sing from the heart of a woman who lived one of the deepest love stories of our time. "As a child I had private ideas and private dreams I never told anybody about," she says. Still, "I never expected the kind of success I've gotten. I haven't accepted it totally. It comes over me more—the disbelief—when I'm home. Everybody knew me and grew up with me, so there's nothing special about me now." After they hear "June Carter" open her mouth, I tell her, I doubt there will be anybody left who really believes that.