Joey King couldn't keep up with Nicole Kidman's intense butt workout while filming A Family Affair

Nicole Kidman plays Joey King's mom in A Family Affair, so it's only fitting that she passed down some life-changing advice to her costar on set.

During a recent appearance on The Tonight Show, King detailed working with the Moulin Rouge star and Zac Efron on their new Netflix rom-com. "I obviously love them so much, but I had never met them before," King said of her castmates. "They're the nicest people. And Nicole especially is such a sweet and silly, fun person. She's such a girls' girl."

King added that she was particularly impressed by Kidman and Efron's athletic physiques. "They're both so hot," she observed. "They're like the fittest people I've ever met. So I was inspired by them, so I became a bit of a tubular gym rat on the set of A Family Affair."

The actress went on to explain how Kidman transformed her workout routine. "Nicole was the one who taught me," King said. "It was this day on set, and I was like, 'I haven't worked out today,' and she was like, 'Let's work out right now!' and I was like, 'We're in jeans!' But she taught me the most epic, awful butt workout I've ever learned in my life."

In fact, she struggled to keep up with Kidman's fitness regimen. "It was so intense," King said. "And I was like, 'I'm a youngster, I can hang. I'm young, I'm agile.' I thought I could hang. But I couldn't hang. It was all these donkey kicks, and the rainbows and the fire hydrants and all these other — you have to keep your leg in the air for like 12 years. It's so hard. It was so insane."

The Kissing Booth star said that she still employs the routine. "I do it still, because it's the most effective workout," King said. "So I send [Kidman] photos when I do it, I'm like, 'Call an ambulance! Love you.'"

Elsewhere in the interview, King said she grew up as a massive admirer of Efron's work, and that he was "so gracious" about her fandom.

"I would sing [High School Musical] on set," she recalled. "And he would sing along with me. And he was, again, so fit. He helped teach me about my protein intake. He's very helpful with the wellness tips, like, 'What's your plasma like?'"

A Family Affair is now streaming on Netflix. Watch the full conversation between King and Jimmy Fallon above.