(CNN) -- Hollywood loves biographical films, and movies about country music stars are almost a genre within that genre, from "Coal Miner's Daughter" (Sissy Spacek gives an Oscar-winning performance as Loretta Lynn) to "Great Balls of Fire" featuring Dennis Quaid's sweat-drenched performance) to "Sweet Dreams" (the fine Jessica Lange as Patsy Cline).It makes a lot of sense: Country music, by its nature, features themes of heartache, survival, and love lost and won, and the musicians -- who usually suffered all the above -- make for great drama.Now, with "Walk the Line," the life of Johnny Cash and his wife June Carter Cash has been put under the microscope. Their story, and the film, does not disappoint.The late June Carter Cash -- she died in May, 2003, and a heartbroken Johnny followed her just five months later -- said her husband had two personalities: Cash the wild, dark man and John the honest and vulnerable man. He was a boy whose father blamed him for his brother's tragic death, and a man who battled fierce demons.Thanks to Joaquin Phoenix's terrific performance, "Walk The Line" captures this dual persona brilliantly. Phoenix -- who had never even played guitar before getting this role -- doesn't imitate Cash. He just gets Cash, in ways great and small. When he steps up to the microphone and says, "Hello, I'm Johnny Cash," you believe him. This performance will undoubtedly win him an Oscar nomination.The same can be said for Reese Witherspoon, who plays June. Director James Mangold made a bold move in casting the petite blonde actress mainly known for lightweight comedy, and she delivers.June Carter Cash's profile was much lower than her husband's, so Witherspoon had a little more leeway in terms of portraying this strong woman, who in many ways was just as complex as her husband. But what she does with a glance, with a line, with a look at the man who would be her husband -- the little things say so much. June met Johnny when they were both married to other people and stuck with him through his drug addiction and wrenching emotional ups and downs.This role should make Hollywood sit up and notice there is more to Witherspoon than meets the eye.The supporting performances are just as good: Robert Patrick as Cash's bitter, bile-spewing father; Dallas Roberts as Sun Records founder Sam Phillips; Shooter Jennings playing his father, Waylon.Equally astounding is the music -- not just the songs themselves, but the fact that the actors do their own singing and playing. Producer James Keach and Mangold were both adamant that whoever played the Cashes would do their own singing. If you wanted to hear Johnny and June Cash sing, you could buy a CD, they believed. It was a gutsy move, but it's instrumental in making a very good movie into a great one.Phoenix growls and Witherspoon purrs and their duets are absolutely wonderful. T Bone Burnett ("O Brother, Where Art Thou?") was the executive music director; he deserves plenty of credit for making the music work.Mangold's direction is excellent, and the script -- by Mangold and Gil Davis -- captures the complex characters without relying on biopic clich�.Bottom line: Get in line to see "Walk the Line."