Mark Harmon, a hero on-screen and off

Mark Harmon as Special Agent Leroy Jethro Gibbs in the procedural series "NCIS," now in its 10th season.

(CBS News) Mark Harmon gets to the bottom of difficult cases on the hit CBS series, "NCIS." After a decade in the role, he's still at the top of his game, as Tracy Smith shows us in this Sunday Profile:

He's arguably the biggest star on TV. But what makes Mark Harmon's story truly remarkable are the things he's done when no one's watching -- things he rarely talks about. More on that later.

Here's what millions already know: Harmon's show, "NCIS," the police procedural that airs on this network, happens to be the most-watched show on television.

It's not flashy or edgy, but reliable, in that the good guys always win -- and funny, in a slay kind of way. It's also durable: "NCIS" is just now wrapping its tenth season.

When the show started, how long did Harmon think this ride would last? asked Smith.

"I think all actors think their ride is going to last forever," he replied. "We're kind of naïve -- or I am, anyway."

On-screen, the 61-year-old Harmon is Agent Leroy Jethro Gibbs, lead investigator and man of few words. Off-screen, his castmates say, he has pretty much the same strong, silent style.

When asked if Harmon is anything like Gibbs in real life, Pauley Perrette responded, "I would say yes-ish."

Cote del Pablo said, "He's a really pleasant person to be around because there's none of that me me me me me me me. It's kind of nice."

Michael Weatherly called Harmon "a pretty ordinary guy for someone who's the biggest TV star in the world."

Mark Harmon (second from right) as Special Agent Gibbs, with Sean Murray (left) as Timothy McGee, Pauley Perrette as Abby Sciuto, and Joe Spano as FBI Agent Fornell, in "NCIS."

The show -- his show -- draws about 22 million viewers a week. But Harmon won't take any of the credit for that -- or really much of anything.

Why not? "Eh, it's too early," he told Smith.

"But that's you as a person - I mean, that's not just this show I'm talking about, you never take the credit," Smith said.

"Well, yeah, because I don't believe it's just me. You know, I believe we all do it together here," he said.

Mark Harmon and Pam Dawber, at the 2002 Creative Arts Emmy Awards at the Shrine Auditorium in Los Angeles, Sept, 14, 2002.
Kevin Winter/ImageDirect/Getty Images

The team concept has always been important to Mark Harmon: the son of Heisman Trophy winner Tom Harmon, Mark grew up tossing a football -- and wound up as starting quarterback at UCLA.

After graduation, he tried his hand at jobs as diverse as carpentry and corporate sales, and he took acting classes at night.

"When I told my mom and dad that I was going to really try to go 100 percent and be an actor, I mean, my parents were supportive, but they thought I was out of my mind," he laughed.

"Did their minds change as time went on?" Smith asked.

"Well, eventually, yeah," he replied.

As an actor, he's always been willing to take a risk: As Dr. Bobby Caldwell in "St. Elsewhere," he was one of the first prime time characters to contract AIDS; and in the highly-regarded TV movie "The Deliberate Stranger," he was eerily believable as serial killer Ted Bundy.

"In some ways, the Bundy role was a stretch," Harmon said. "But I was glad to put that one away when it was done."

Harmon seems more comfortable playing characters on this side of the law, like a Secret Service agent in "The West Wing."

In a job where, as an actor, he is expendable, Harmon is drawn to things that last.

A world away from the "NCIS" set, on the windy streets of Oklahoma City, he has quietly made a difference.

Harmon and a small group of friends, like Dr. Michael Wright, have expanded the Oklahoma City Indian Clinic, a medical facility which now serves 3,500 kids.