Together for more than forty years, Goldie Hawn is getting candid about why her relationship with Kurt Russell is so successful.

On Friday, while celebrating the 20th anniversary of The Goldie Hawn Foundation and MindUP, the actress revealed a key element to their love story.

"You have to have good sex," the 78-year-old said bluntly in an interview with E! News.

KURT RUSSELL, GOLDIE HAWN KISS DURING ROMANTIC ASPEN GETAWAY AFTER 40 YEARS TOGETHER

"Because sex is something that connects you and creates more belonging. People who have healthy sexual relationships usually last a lot longer. But it's not just because of the act, it's because of the warmth and the intimacy that it creates," she explained. 

"You have to be nice to each other upon occasion," she teased.

Although Hawn said having that intimacy is important, it's also the time spent recognizing your differences that really makes a relationship stick.

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"You know you're not the same person," the "Overboard" actress said. "You don't think the same way oftentimes. And you have to accept that, but you have to measure are we having fun, or is this something we want to do? Do we have laughs together? Do we share certain things? You don't have to share everything."

"We have a lot of expectation, I think, around relationships, but you have to like the person. That's very important."

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Russell, 73, had his own take on why he and Hawn had seen such longevity in their relationship.

"Most people that have been together for a long time, I think, share one thing in common," he told the outlet. "And that is that after that many years together, you will have experienced just about everything there is to experience together and understand all the ups and downs and what it means."

"I just love the fact that we met and [we're] still doing it. We still like being together." 

The couple originally met on the set of their film, "The One and Only, Genuine, Original Family Band," but didn't get together until years later. Work brought the duo back together in 1983, for their film "Swing Shift." 

Hawn and Russell, who each have their own children from previous marriages (Kate and Oliver as well as Boston, respectively), share one son together, Wyatt. 

During a March appearance on Dax Shepard's podcast, "Armchair Expert," Hawn spoke extensively about her relationship with Russell and the obstacles they overcame as a couple. 

"We don't agree on certain things. He was tougher than me as a parent. … But we have a very, very strong family because of it," she said. 

"That was one of the things that gave us things to talk about," she said of why their relationship stuck. 

The couple is also famously not married.

"I think that if we were married, there probably could have been times, when you go, ‘Oh c’mon. I'm done,'" she told Shepard. Still, Hawn said, "I'm really happy I stayed."

Don Johnson has an easy secret to staying handsome and energetic at 74.

"Marry right!" he told People magazine, adding, "A bad marriage will age you."

The "Miami Vice" star has been married to his wife, Kelley Phleger, since 1999. The couple share three children: Atherton Grace, Jasper and Deacon.

In another interview with People earlier this month, Johnson said the keys to his 25-year marriage to Phleger are simple.

DON JOHNSON SAYS THREE THINGS ARE 'KEY' TO HIS LONG-LASTING MARRIAGE

"Oh, we are just so in tune with each other that there's an unspoken, like, trust and kindness and respect with us. And those are the keys," he told the outlet.

In an interview with Parade last year, Johnson called her "an amazing woman."

"Obviously, she’s a saint. I was with Bob Dylan one time, and I was having relationship issues, and Bob looks up at me and goes, ‘Well, see, love is about kindness, trust and respect, isn’t it?’ And that’s the embodiment of Kelley," Johnson said about what makes Phleger perfect for him. "It’s kindness, trust and respect, and we have that with each other. We’re lovers and friends and it’s fun. She’s an amazing woman."

He is also father to a son from his relationship with Patti D’Arbanville, Jesse Wayne Johnson, and daughter Dakota Johnson, from his marriage to Melanie Griffith.

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Johnson and Griffith married when she was 18 in 1976, after first meeting when she was 14. They divorced in November that same year.

They married again in 1989 and remained together until 1996. Johnson also adopted Griffith’s son from her relationship with actor Steven Bauer, Alexander Bauer, per People.

Being a dad is another one of Johnson’s secrets to looking and feeling young.

"You have to exercise, and do it a lot," he told the outlet. "How do you think I got six kids?" adding jokingly, "That I know of, anyway!"

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Johnson’s youthful energy is being put to work, with headlining two projects, Netflix’s "Rebel Ridge" and the new Hulu series "Doctor Odyssey."

Earlier this month, Johnson reflected on the ups and downs of fame during his career-making success on "Miami Vice."

"For a long time, I couldn’t even go anywhere," he told People. 

He continued, "I had to isolate. The [women] know where you are. We had security 24/7."

The intense fandom reminded him of what Elvis Presley must have gone through at the height of his fame.

"One day, I reflected on Elvis and thought, ‘Isn’t this kind of the way Elvis lived and died?’ Not that I’m comparing myself to Elvis, but in terms of him not being able to have a personal life. So I learned to manage it and vowed not to let it imprison me," Johnson said.