Reese Witherspoon should have been given her own movie. As June Carter in “Walk the Line,” Witherspoon redefines the notion of “supporting actress” by playing her submissive role with exceptional brilliance. Her character is not only the apple of Joaquin Phoenix’s eye, but also the moral pillar and source of inspiration on which the entire film is based, saving it from being just another formulaic biopic.

Those who see director James Mangold’s “Walk the Line” unfortunately will need to sit through the first 30 minutes, a dry spell lacking so much as a flicker of Witherspoon’s cunning visage. Still, Joaquin abounds, impersonating Johnny Cash with skill and precision, but never even approaching the kind of dramatic radiance that Reese brings to her role. When she does appear, though, the film suddenly becomes a one-woman show. All eyes and ears are on this regal brunette songstress as she alternates between public and private personae, complete with an authentically Southern lilt (Reese is a Nashville native).

Some may complain that Reese does not do her job in “Walk the Line” — her job being to play second fiddle to Phoenix’s legendary Cash. They would be right. Reese takes center stage, stealing every scene, topping every line with a perfected glow that takes attention without begging for it. She is, perhaps, a better June Carter than June Carter. As the object of Cash’s affection, though he and she are both married with children, she provides a muse for his music. When he writes songs, he does so for June. He even woos her into going on tour with him multiple times, just to get closer to her. His persistence, though, is nearly always met with stern resistance, as June is a careful observer of how she is viewed by her fans and her peers.

Her steadfast morality becomes an important aspect of the film, especially as Cash’s life and career start to tumble out of control. From drug addiction to alcoholism, Cash stumbles through a heavy portion of the film with a drunken grimace, ruining concerts and relationships faster than he can sing about them. June becomes his backbone, picking his sorry ass up after each failure, enduring miles of criticism for sticking by him. That is perhaps why her moments of weakness produce such devastating effects — when her face crumples up in tears, the world itself seems to be fall apart. When she is insulted for getting a divorce, her apology conveys a sense of regret so vulnerable it is heartbreaking.

Admittedly, there are admirable aspects of “Walk the Line” that Reese Witherspoon cannot take credit for — well, not full credit. Clearly, Joaquin Phoenix’s performance is solid. He becomes Johnny Cash similar to the way that Jamie Foxx became Ray Charles (“Ray,” 2004). And the music, actually performed by the stars — not lip-synced — is perfect. When Phoenix sings Carter’s song “Ring of Fire” near the end of the film, his voice is so close to Cash’s original recording that one feels the need to call an exorcist.

To top it all off, the involvement that Johnny Cash and June Carter had in making this film should not be ignored. The two died within a few months of each other, but not before approving the shooting script and the actors who would portray them. Cash is said to have hand-picked Phoenix for the role. And while Carter’s approval is not so certain, there can be little doubt that Carter would be immensely proud of Witherspoon’s superlative performance.

“Walk the Line” represents, so far, the best of her career — relatively long for an actress of 30 — and it should not go without recognition. People are already mentioning words like “Oscar” and “acceptance speech,” but what would be an even better reward would be the name Witherspoon on every Hollywood film worth making. And, of course, it should be listed first — it just makes more sense that way.