As a child, Diana hoped to be a ballerina. That dream died in her teens, when she reached her full height of 5'10"—too tall to be a classical danseuse. But in October 1985 she asked well-known British dancer Wayne Sleep to choreograph a routine for her to perform at the Christmas benefit for the Friends of Covent Garden at the Royal Opera House. It was to be "a surprise for her husband," Sleep says. Diana chose Billy Joel's "Uptown Girl," one of her favorite songs, and secretly trained a few hours a week at various studios.

During the first two-thirds of the benefit, on Dec. 23, Diana sat beside Charles. Then she quietly left the royal box, slipped out of her red-velvet dress and changed into a clingy, cream-satin gown.

Sleep was first onstage. "There was a big round of applause," he recalls, "and I thought, 'Oh, you ain't seen nothing yet.' " Indeed, the crowd gasped when Diana made her entrance. "I told her to walk on for eight counts and then stand for eight more, because they are not going to believe it," Sleep says. "Every step we took was followed by a huge round of applause."

But Diana seemed to be performing for an audience of one—her stunned husband, whose box was right over the stage. Diana's routine was "among the most provocative and sensuous pieces of theater you are ever going to see," says Reg Wilson, Covent Garden's official photographer that evening. "She kept looking up at Charles. There was an enormous sense of fun between the two of them.... He was very happy and smiling the whole time."

At the end of the four-minute routine, the duo took eight curtain calls, and Diana begged for a reprise. "I said no," says Sleep, "because then they would start nitpicking. She's a good dancer, but she isn't a professional. She started to do it again, and I had to drag her off. She loved it."