Billy Crystal thinks "humour and trust" are the keys to a successful marriage

He told Fox News Digital: "Humour and trust and constantly be interested in what each of us are doing and respecting our separate lives and who we are. But our common goal was always just to raise a beautiful family, which we have and continue to do. And just respect each other. And she's an extraordinary person.

"I'm lucky. I was 18. She was 17. And here we still are."

The 'When Harry Met Sally' star previously revealed that he and Janice had met in 1966 while he was working at a summer camp in Long Beach, New York and it was love at first sight.

Speaking on the 'WTF with Marc Maron' podcast, he said: "This girl walks by ... and I said, 'I'm gonna marry her.' And I did, four years later. I loved her so much right away. You know, I don't know if you've ever been in a long-distance relationship - they don't work out."

The 'Before' actor also recently noted how much their relationship changed after becoming parents.

He told People: "When I first became a dad back in [1973], Janice was working, and I was starting my career as a stand-up at night. So, during the day, I was 'Mr. Mom' and that changed my life totally.

"The responsibility as a 25-year-old with an infant that can't get through the day without you," Crystal continued. "When you learn that you can love something much more than yourself, it was the greatest experience in my life as a person.

"That's been my total effort of my life is to leave the world with two amazing daughters - grown women now, mothers of their own," he explained. "And that they then teach their children, our four grandchildren, what it means the responsibility in our world, which is changing constantly."