Like so many times before, Prince Charles and Princess Diana were sitting in their Royal Box at the Royal Opera House in London’s Covent Garden one evening in December 1985. On this particular night, the mood was lighter than normal since it was a private event to thank the patrons who had supported the Royal Ballet and the performances were more offbeat.

Toward the end of the evening, Diana slipped out of the Box. Charles reportedly assumed she had gone to use the ladies’ room, so he continued enjoying the program as dancer Wayne Sleep took the stage to tremendous applause.

But then a surprise guest joined him on stage — someone whose appearance shocked every single person in the opera house: Princess Diana was up on stage dancing with Sleep.

“The audience gasped when Diana appeared, as if they’d all taken one huge breath,” Sleep told The Guardian in 2017.

What they didn’t know at the time was that Diana’s dance had been a long time coming. She had been rehearsing with Sleep and wanted to surprise her husband with the performance.

In an age before cell phone cameras and social media, it was never meant to go too far beyond those sitting in the opera house that night. But now it’s famously known as one of the princess’ most daring moments.

Diana initially approached Sleep for dance lessons

Diana had long been impressed by Sleep. After all, he was once the principal dancer in the Royal Ballet and also holds the world record for doing a jump with 12 feet switches called the entrechat-douze, which he did in 1973.

“Our paths first crossed when she approached me for dance lessons in the early 1980s,” Sleep told The Guardian. “Diana loved ballet, but she also wanted to learn jazz, tap, and contemporary. Sadly, I couldn’t teach her, because I was away on tour so much.”

But Diana’s dreams of dancing never went away. And when she got the idea of performing at the Royal Opera House’s annual event, she reached out to him again.

This time, he was able to commit.

The princess wanted to surprise Charles with her performance

The young royal already had a clear idea in her head of her plan — she wanted to dance on stage alongside Sleep to Billy Joel’s “Uptown Girl.” But it had to be a truly zipped up secret since she didn’t want her husband to find out about it until she stepped on stage.

“We met in a rehearsal studio in west London,” he recalled to the paper. “She was in leg-warmers and a leotard. My first thought was, she’s too tall to dance with me, I’ll be a laughing stock: I’m 5-foot-2 and she’s 5-foot-11.”

Sleep’s height had long been a part of his career as a dancer. In fact, he was the shortest male dancer ever in the Royal Ballet School and had many roles specifically written for him because of his stature.

“I soon realized she had a good sense of humor, and that we could have some fun with our height difference,” Sleep said as they started working together.

The routine they came up with was packed with playfulness, while some even considered it tongue-in-cheek. “At one point, I pirouetted and she pushed me down,” the dancer said. “Then I carried her across the stage. I remember thinking, ‘Don’t drop the future Queen of England.’”

Thankfully it went off without a hitch. “The routine had a bit of everything: jazz, ballet, even a kickline,” Sleep said.

READ MORE: Princess Diana and Prince Charles: A Complete Timeline of Their Relationship

Diana didn't want to bow to Charles

The princess was absolutely delighted at the end of their choreography. She couldn’t believe that she had pulled off such a surprise. “She loved it, but was most thrilled we’d kept it secret from Charles, and our rehearsals away from the paparazzi,” Sleep said.

At the end of the performance, as per protocol, the performers always bow to the Royal Box. That’s when things got a little murky.

“I said, ‘By the way, you’ve got to bow to the royal box,’” Sleep said when he was on Big Brother. “And she said, ‘I’m not bowing to him — he’s my hubby!’ And so I made her turn and bow, and as we ran off she went, ‘You won’t get your OBE [Order of the British Empire] that way.’”

Princess Diana and Wayne Sleep at Sadler's Wells Theatre
Princess Diana and Wayne Sleep at Sadler’s Wells Theatre
Photo: Tim Graham Photo Library via Getty Images

Prince Charles reportedly wasn’t happy with the surprise

While the audience was delighted, things apparently didn’t go over so well inside the palace.

Some headlines said that Prince Charles felt like Princess Diana was “showing off,” while others said that it made him “uneasy” that she performed without him — and still others suggested that he may have thought the dance was too “sexy.”

Charles himself was no stranger to the follies that could happen on stage at this particular event — and in fact, he clearly had his own sense of humor too. The previous year, he and Diana got on stage together and did a Romeo and Juliet-type skit that involved him singing an ad jingle “Just One Cornetto,” according to Tina Brown’s book The Diana Chronicles.

But apparently, this was different — a line had seemingly been crossed.

Diana and Sleep forged a friendship, bonding over their loneliness

Despite what may have happened between the royal couple, Diana loved the freedom she felt when she was on the stage — it was such a far cry from her normally stiff lifestyle.

A few days later, Sleep received a letter from his star pupil. “Now I understand the buzz you get from performing,” she wrote.

Pulling off such a tremendous feat bonded the two and they became tight friends. Sometimes she would go to his shows and visit him in his dressing room afterward and other times she would have dinner at his south Kensington flat.

“She would kick off her shoes and we’d have a giggle, talking about this and that: her kids, my work, nothing controversial,” Sleep said. “I’d cook, and she’d wash up. Her security would wait across the road, in my studio.”

He thought that perhaps they got along because they were both “loners” who carved out their own paths. He also helped her appreciate the mundane things in life that she wasn’t able to as such a public figure. “I would buy her silly postcards,” he said. “She loved them, but couldn’t go into shops and buy them herself.”

But eventually, they grew apart — and Sleep wonders if it might have had to do with a photo leak that happened a decade after their decade.

“In 1995, these pictures – taken by the Royal Opera House’s photographer – appeared in the tabloids,” he told The Guardian. “No one knows for sure how they got hold of them. Diana got suspicious – by then, she didn’t know who to trust anymore – and we drifted apart.”

READ MORE: Princess Diana and Queen Elizabeth II: The Tumultuous Relationship Between the Royals

Dancing Diana made headlines twice in 1985

The timing of her surprise dance came just a month after Princess Diana had famously hit the dance floor on the other side of the Atlantic.

At a White House party in November 1985 during Charles and Diana’s royal tour of the United States, Diana showed off her twinkle toes by accepting actor John Travolta's invitation to dance.

They ended up on the dance floor for 15 minutes, which he now calls “one of the highlights of my life.”

Princess Diana trying on a kimono that she received as a gift at Nijo Castle, Kyoto, during an official visit to Japan on May 1986