Robert Downey Jr.'s wife Susan reveals the secret to their 18-year-marriage

Robert Downey Jr.'s wife Susan Downey has revealed the secret to their 18-year-marriage.

The film producer has revealed she and the Oppenheimer star have a rule to ensure they spend enough time together amid their busy schedules.

"We do have a two-week rule, which often feels too long, but we don't go more than two weeks without seeing each other and the family being together," Susan, 50, told People in a new interview.

She then joked that she and Robert, with whom she has children Exton, 12, and Avri, nine, "prefer to be a travelling circus when we can be."

The Sherlock Holmes producer then acknowledged that their jobs can often be "unpredictable".

"You keep the basic rule of two weeks, and then you don't try and think too far ahead, because so much of what we do is oddly unpredictable," she told the outlet. "You just never know."

"Someone might get sick, or there might be a shutdown, or this or that," she continued. "So you just have to have some basic things in place, and then you have to be willing to flow with the rest of it."

The couple first met in 2003 while filming the thriller Gothika, which Susan produced and Robert starred in alongside Halle Berry and Penélope Cruz. They then tied the knot two years later in 2005.