"Walk the Line" gets off to a promising start, sure to raise goose bumps, at the gates of Folsom Prison in 1968. The camera makes its way slowly into the jail, and 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. 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. 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.

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." 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 addiction to speed, his divorce and eventual marriage to Carter -- it's a more plodding affair.

-- Ann Hornaday

Joaquin Phoenix and Reese Witherspoon share chemistry -- at least while on stage -- as Johnny Cash and June Carter.