Catherine Zeta-Jones has a "very open" relationship with Michael Douglas. Picture: AP

Catherine Zeta-Jones has a "very open" relationship with Michael Douglas.

The Mask of Zorro actress says the secret to her long-standing romance with the 74-year-old acting legend is that they are both very "open and honest".

She said: "Both Michael and I have a very open relationship. When you have kids that didn’t ask to be born into a world where [the marriage is] scrutinised or looked into. You’ve got to be open and honest and share things that probably people at the drug store wouldn’t share over the counter."

However, the 49-year-old actress insists things aren't "rosy" every single day. She added: "First of all, I’m so happy that we’ve made it to be 20 years together. It’s been great. But I think it’s just unfathomable to me that you would be with one person for 18 years and things are not rosy every day. You know? They’re just not."

And Catherine likes to talk about things openly and "clear the air". She shared: "I’m a big European kind of family girl, and so everything is shared around the table. Issues with my daughter if she’s feeling insecure - she’s a teenager and girls can be mean. My son’s going off to college. And then you just clear the air. Everyone knows what’s going on. There are no surprises."

The Ocean's Twelve star's parents - Patricia and David - have been married for over five decades and Catherine thinks it has been good to have them as inspiration for her own marriage. 

She told The Today Show: "My mother and father are married for 52 years and they’re a wonderful blueprint for me because I’ve seen the good, the bad and the ugly with them, and they’re still going strong."