Mark Harmon on His 31-Year Marriage to Pam Dawber: 'I'm Proud to Be Married to Her'

After 40 years in the spotlight — and 16 years on NCIS — Mark Harmon is opening up about what matters most: his marriage, his kids and the value of a hard day’s work.

Sitting down with PEOPLE for this week’s cover story, Harmon, who plays Navy investigator Leroy Jethro Gibbs on the hit CBS show, reveals the secret to his 31-year marriage to actress Pam Dawber: maturity.

“We were both in our thirties when we got married, so hopefully the stupid stuff we did earlier,” he says. “That’s probably the closest I have to what the key is.” (Harmon and the Mork & Mindy star, both 67, share two children: sons Sean, 30, and Ty, 26.)

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One other thing that helps them click? Their differences. Harmon says Dawber was actually opposed to him taking what would turn out to be a beloved role — the goofy teacher Freddy Shoop in 1987’s hit comedy Summer School.

“I wanted to do it just because I was excited to work with [director] Carl Reiner,” he says. “That was a game changer for me, like, ‘He thinks I can do this?!’ and that meant a lot. But Pam said, ‘Don’t do that. Don’t do this movie.’ We don’t generally like the same thing, and that’s been healthy for us, actually.”

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Asked about the distinctive ring — in the shape of an eagle head — on his wedding finger, he explains, “We think it was my grandfather’s, because it was in a box of things that my dad left.”

“I wear it or a few others I have because when I’m not shooting, I like to make sure I have something on my ring finger, because I’m proud to be married, and I’m proud of who I’m married to,” he says. “I’m just so proud of her.”