DEFINING 'WALK THE LINE'.

Byline: Bob Strauss Film Critic

YOU'RE DOING something right when you can convince the world that you're Johnny Cash - and half your performance is doing your own singing.

Joaquin Phoenix pulls it off with great, sweaty aplomb in ``Walk the Line.'' And Reese Witherspoon is even better as the towering country star's soul mate, June Carter (and while no one can perfectly imitate the Man in Black, she actually sings better than the distinctive-voiced Carter did).

Together, Phoenix and Witherspoon enact one of the year's great screen love stories. He's all brooding, self-destructive desire, and she's a sassy Nashville princess with an unerring moral compass that we'll just have to assume Carter's first two husbands couldn't abide. Neither actor ever loses the scent. But while their delicious, long-haul courtship and scintillating duets fill ``Walk the Line'' with romantic pleasure, the film doesn't quite cut it as a definitive biopic.

Co-written by director James Mangold (``Girl, Interrupted,'' ``Identity'') and Gill Dennis, reportedly with the enthusiastic cooperation of Johnny and June before their 2003 passings, the film follows standard music bio procedure for most of its length.

We see J.R. Cash's Arkansas sharecropper childhood, complete with the death of a beloved brother that will supply guilt demons for the film's entire running time (just like in that jazzier, better-rounded Ray Charles movie). Then there are the early career struggles, including dubious dramatizations of how he wrote such hits as ``Folsom Prison Blues.''

We also get an extended examination of Cash's amphetamine dependence - so extended that the movie's last hour feels composed entirely of Johnny blitzed out or hunting for June's hidden stash. Well, it's not entirely that; June also says no to about 100 of John's marriage proposals.

Between all the boilerplate, though, Mangold and company dish out an exuberant celebration of pop-music history.

Cash's audition at Memphis' landmark Sun Records is a riveting scene, especially once legendary producer Sam Phillips (a terrific Dallas Roberts) goads him into dropping the country gospel shtick and playing something from the heart. Then there are the exciting and hilarious midnight-ride rockabilly tours with Elvis (``One Tree Hill's'' Tyler Hilton), Jerry Lee Lewis (Waylon Payne), Waylon Jennings (played by his own son Shooter) and, for some reason, corn-pone comedian June all crammed in the same sedan.

There's also very good support from Robert Patrick as Johnny's unforgiving daddy (the former Terminator 2 is making a career out of this; he played Vernon Presley on the recent ``Elvis'' miniseries). And Ginnifer Goodwin (``Mona Lisa Smile'') pulls off the near-impossible task of making us feel for Johnny's first wife, Vivian, who is put in the thankless position of keeping the future golden couple apart.

But while we can't get enough of Johnny doggin' June (until we do), it seems a bit perverse to focus so much of the film's 1960s section on that story line and the drugs. Cash's complex struggle between faith and sin is barely touched on, and his cultural eminence as a politically progressive patriot - in an era when the country was at partisan extremes - is completely overlooked.

The film climaxes with Cash's 1968 comeback concert at Folsom, and both Phoenix and Mangold give it all they've got. Throughout the film, the young actor amazingly evokes Cash's performing spirit while managing to sing the songs his way.

Meanwhile, Witherspoon finally recaptures the dramatic gravity that made her such an exceptional child actor. But the grown-up comedy star is never too far away, which is perfect for June. Considered the least gifted of a musically accomplished family, she got by telling jokes until Johnny the genius saw and brought out her potential.

And it still took 'em a dozen years to get hitched. ``Walk the Line'' may not be the most innovative musical biopic, but they sure ain't makin' love stories like this one anymore.

Bob Strauss, [phone]

bob.strauss(at)dailynews.com

WALK THE LINE - Three stars

(PG-13: drug use, language)

Starring: Joaquin Phoenix, Reese Witherspoon, Ginnifer Goodwin, Robert Patrick.

Director: James Mangold.

Running time: 2 hr. 10 min.

Playing: In wide release.

In a nutshell: Joaquin Phoenix and Reese Witherspoon are terrific - and sound great singing with their own voices - in this biography of Johnny and June Carter Cash. But while it's got a great love story, the film overlooks important aspects of the Man in Black's life and career.