"Walk the Line" gets off to a promising start, sure to raise goose bumps, at the gates of Folsom Prison in 1968. While an unseen band's rave-up builds, the camera makes its way slowly into the jail, where the gathered convicts are at a fever pitch waiting for their hero to take the stage. Finally, after an excruciating, exhilarating eternity, we see Johnny Cash -- played, in an uncanny performance, by Joaquin Phoenix -- as he drinks a glass of water and contemplates a buzz saw. We're hooked, and he hasn't sung a note.

Unfortunately, for all its good music and admirable vocal impersonations, "Walk the Line" slides -- very, very slowly -- downhill from there, as writer-director James Mangold hews to the hoary conventions of an increasingly tired Hollywood staple. Boy nurtures nascent talent, boy suffers primal loss, boy meets girl, boy meets drugs, boy loses girl, boy kicks drugs, boy gets girl, boy is redeemed. From "Ray" to "Beyond the Sea" to last week's "Get Rich or Die Tryin' " and now "Walk the Line," each has hit exactly the same notes, with only a slightly different order or permutation.

And, as rote as the narrative has become, the audience's own expectations have grown just as blinkered. The modern-day biopic is less a movie than a parlor game, as viewers watch out for historical gaffes and rate the actors on their impersonations rather than their performances. (Once in a while they deliver both, as Sissy Spacek did in "Coal Miner's Daughter," perhaps the last of the great examples of the genre.)

So, yes, Phoenix does an impressive job of embodying Cash, from the sexy sneer to the way he held his guitar. What's more, he nails the singing voice, whose rich bass notes and raw, raspy breaks seemed at once ancient and blisteringly new. Reese Witherspoon, who tackles one of her few dramatic roles as June Carter, isn't quite as convincing offstage as the little sister of the legendary Carter Family, but once she and Phoenix share the microphone, she proves to have a surprisingly strong and sweet voice. (If Fred Astaire gave Ginger Rogers class and she gave him sex, as the saying goes, then "Walk the Line" suggests that Johnny Cash gave June Carter soul and she gave him flash.)

Indeed, the musical sequences are the best thing about "Walk the Line," and luckily they are plentiful in a film that focuses on Carter and Cash's virtually lifelong love affair. When the two are onstage together they create the heat that in real life inspired such scorching classics as "I Walk the Line" and "Ring of Fire." (The music was produced by the always dependable T-Bone Burnett.) When the film takes viewers through the paces of Cash's biography -- his youth picking cotton in Arkansas in the 1940s, the loss of his older brother in a horrifying accident, rejection by his father, his early marriage, his discovery by the great producer Sam Phillips at Memphis's Sun Records, his ascent through the Sun ranks (he opened for Elvis and Jerry Lee Lewis), his addiction to speed, his divorce and eventual marriage to Carter -- it's a more plodding affair.

Part of the problem is structural. Unless they're going to completely explode the genre, as Gus Van Sant did this year with "Last Days," or address it head-on, as Kevin Spacey at least tried to do in "Beyond the Sea," directors are hemmed in by the constraining tropes of the biopic form. By those rules, "Walk the Line" is predictably episodic, with each high and low point of Cash's life given its dutiful tableau vivant. But didn't we already see Sam Phillips look with amazement at the bright young star who just wandered into his storefront studio? (Oh, right, that was Elvis in whatever TV movie was just playing on cable.) Didn't we just see a spurned woman in a wasp-waisted dress throw something at her straying man? Sorry, that was Sandra Dee. Or Ray Charles's wife. Or somebody. After more than two hours, the reenactments grow numbing. In fact, the epic sweep and mythic romance of Cash's life were probably captured with more eloquence and passion in the four-minute video Mark Romanek directed for "Hurt," the Trent Reznor song Cash recorded not long before he died in 2003.

The other chief problem in "Walk the Line" is the performances. Although Phoenix eventually succeeds in disappearing into Cash the way Spacek did into Loretta Lynn, Witherspoon never once lets viewers forget that they're watching her and only her. She may have it in her to be a good dramatic actress, but she might have been better advised to try a smaller canvas before tackling such a monumental role.

Meanwhile, viewers are continually taken out of the movie by distracting cameos featuring overeager young actors playing Elvis and Jerry Lee. The most touching and indelible relationship in the movie turns out to be one that is represented only obliquely on-screen: Cash's friendship and collaboration with Bob Dylan, who hovers over the proceedings like a benevolent ghost. You've got to wonder -- and shudder to think -- who will play Cash in the Bob Dylan biopic.

Walk the Line (136 minutes, at area theaters) is rated PG-13 for some profanity, thematic material and depiction of drug dependency.

Joaquin Phoenix is convincing as Johnny Cash, but Reese Witherspoon falls short as June Carter.