Reese Witherspoon is the actress most likely to inherit Meryl Streep’s acting crown, outrank Julia Roberts as Hollywood’s highest-paid female star and pick up an Oscar tonight for her performance as June Carter Cash in Walk the Line.

Not bad for a Southern-born actress of 29 used to playing an all-American dumb blonde. She is not as crazy as two squirrels in a box, as they say down Tennessee way, and there was no broken home — just a happy family tracing their roots back to Scotland.

For good measure, she has stayed married for six years to the actor Ryan Phillippe, who is in the Oscar-nominated Crash, and they have two children, Ava Elizabeth, 6, and Deacon, 2.

Growing up in Nashville made her perfect casting for the singer wife of Johnny Cash in Walk the Line, a role for which she seems to have been rehearsing since childhood. At the age of nine she portrayed Maybelle Carter, June Carter’s famous musician mother, in her school play.

Playing the daughter was more demanding. It required six months of learning how to sing and play the autoharp, five or six hours a day, before studying videos of Carter in concert.

She and her co-star Joaquin Phoenix, who plays Cash, were put through a musical “boot camp” with a string of voice coaches. Witherspoon struggled with June’s lilting inflections.

“When we started we sounded bad,” she told The Times recently. “We were terrified someone would get our first recordings and put them on the internet. The longer we worked on it the better we got.”

Walk the Line tells the story leading up to the Cashes’ 35-year marriage — the rocky years of former relationships, amphetamine addiction and terrific music. It also ranks as Witherspoon’s first fully adult role, a sometimes harrowing account of tangled love that puts her in a different league from the ditzy comedies such as Legally Blonde that brought her to fame.

The result has brought critical acclaim. Winning best actress at the Golden Globe awards has given her the edge over her main rival for an Oscar, Felicity Huffman in Transamerica. Her appearance at the Globes ceremony raised a few eyebrows and will make her the focus of particular attention tonight: it was noticed that the actress Kirsten Dunst had worn the same dress to the award show three years earlier.

However, everyone agrees that success couldn’t happen to a nicer person, whose face resembles an upside-down teardrop with blue-green eyes, long dark lashes and elegant brows. She has a 1,000-watt smile and a peaches-and-cream complexion. “Reese is the actress who just seems too good to be true,” a Hollywood insider said last week. “She is sweet, bubbly and has a loving family around her.”

The New York Times put it better: “She projects a fundamental niceness that makes it impossible to begrudge her all the goodies that flutter into her lap like candy sequins down from Barbie doll heaven.”

Niceness pays as niceness does. It was reported she would get $29m (£17m) for the horror movie Our Family Trouble, exceeding the record set by Julia Roberts of $24m for Mona Lisa Smile, although Witherspoon’s agent cast doubt on the figure.

Strange then, that such an epitome of glamour has revealed that she was so self-conscious about her body that she would never again be seen on screen wearing a bikini.