The secret to a successful marriage? Catherine Zeta-Jones spills how she's stayed happily married to Michael Douglas for 20 years

Catherine Zeta-Jones and her husband Michael Douglas recently celebrated two decades of marriage, and now, the actress is ready to spill their secrets for success.

During an interview with the Wall Street Journal, Catherine divulged that laughter is the most important thing to keep alive in any marriage.

Read more from woman&home:  
• Best mascaras to lengthen, define, and volumize your lashes
• Best jeans for curvy women that are seriously flattering
• Best bras to support, lift and shape your bust

"First of all, we have a lot of fun together," said Catherine. "We've never, ever lost our sense of humor, and we enjoy each other's company."

The two don't have a traditional relationship, and Catherine addressed that, noting that, like any relationship, the two have their "ups and downs."

"My husband is 25 years older than me; that's not a secret. With any relationship, it wouldn't be normal if there weren't any ups and downs," she added.

They also don't follow the typical nine-to-five work schedule that most couples have, which either granted them more or less time together.

"My husband and I spend a lot of time together because, unlike so many couples, we've never had a 9-to-5 job where it's consistent," said Catherine. "We're either on, or we're off. I'm really working 16 hours a day, or I'm not. He's doing the same, or not. So we've had in our relationship huge amounts of time where we've been just us."

Though their work schedules may fluctuate, it's the love and respect of one another's time and space that has allowed their relationship to remain steady and strong. Catherine also noted that "we respect each other's space, and our humor is just long-lasting," which has also helped fuel their relationship for so long.

The Chicago actress and Michael officially tied the knot on November 18, 2000, and share two kids together: Dylan and Carys (Michael also has another son, Cameron, who he shares with his ex-wife Diandra Luker).