There are certain behaviors we’ve come to expect, even demand, from our TV stars: Regularly gracing magazine covers, becoming permanent fixtures on red carpets, making frequent talk-show appearances, signing on for glossy ad campaigns, routinely juggling movie roles and instagramming/tweeting up a storm. Kerry Washington, Sofía Vergara, Melissa McCarthy, Jon Hamm, Neil Patrick Harris, Ellen DeGeneres—hell, even Kim Kardashian—all have held up their end of the bargain. We know seemingly everything, sometimes too much, about them.

And then there’s the curious case of Mark Harmon. He’s the lead of a little CBS drama called NCIS, which returns Tuesday for its 12th season. While it may not be a show you watch, nearly 20 million people tune in every week (8 million more than the average audiences for Scandal and Modern Family), a number that skyrockets to 35 million when you factor in those who watch it in syndication on USA.

NCIS, which stands for Naval Criminal Investigative Service, is not only the third-most popular show on TV (just behind Sunday Night Football and The Big Bang Theory), but was recently named the most-watched drama in the world, with more than 57.6 million viewers globally in 2013. The show, which has spawned two spinoffs (the newest, NCIS: New Orleans, also premieres Tuesday), is now “a billion-dollar franchise,” as CBS Entertainment Chairman Nina Tassler declared this summer—and Harmon is at the center of it.

Those gargantuan figures, both viewers and dollars, make Mark Harmon the world’s biggest TV star. Not that you’ll ever hear the actor admit that, or say anything else personal about himself. Which is just the way he likes it.

Harmon is an anomaly in today’s overshare-first-ask-questions-later pop culture: an anti-celeb. There’s no gushing about the secrets of his 27-year marriage to Mork & Mindy star Pam Dawber (which is more like 270 in Hollywood years), no off-the-cuff speeches about politics or anything else controversial; no statements, in fact, that aren’t in some way related to his show. And his actions speak just as softly as his words: When you search “Mark Harmon” on TMZ, not a single story comes up, which doesn’t even seem possible. He’s perhaps the only person in Hollywood who says he wants his work to speak for itself, and actually means it.

So why hasn’t the actor made more of his stardom, either for good or evil? “One of the big reasons I took this job in the first place, it was about really trying to stay home,” says Harmon. “I had a young family. And the first day on this show was 22 hours. We had a lot of those in the first four or five years here. We’ve learned how to do it better. We’re still doing a 14-hour-day and my wife says, ‘You’re the only person in Hollywood who’s happy about working a 14-hour day!’ I said, ‘Well, compared to where we came from, that’s a huge improvement.’ There’s immensely talented people on this show and one of the nice things about being there is, everybody has a voice. We don’t keep secrets. It’s just an enjoyable job. And I say that, having had them where it’s not.”

That response is classic Harmon: turning everything back into talking about his work. He’s allergic to the spotlight, or at least the notion of having the spotlight to himself. The actor was affable and endearing when we spoke at a CBS party this summer, but whenever he spotted an NCIS costar walking by, he’d ask, “Hey, have you talked with her yet?” and attempted to draw them in. When castmate Pauley Perrette pointed to a nearby red carpet, lined with reporters and camera crews from dozens of outlets, and asked if he would pose for a group picture, Harmon initially thought she’d asked him to do interviews on the press line, and reacted as if she’d suggested he have a root canal.

But that quiet approach has served the onetime UCLA quarterback, now 63, well through his lengthy acting career. He first hit it big as Dr. Robert Caldwell on St. Elsewhere—the hospital heartthrob a decade before George Clooney did it on ER. He made a go of a film career, most successfully in the comedy Summer School, though Harmon tells me that he tried to convince the film’s director, Carl Reiner, to hire Tom Hanks instead, telling Reiner, “I’m not funny!” He returned to TV in Chicago Hope, before landing NCIS, a spinoff from JAG in 2003, playing Special Agent Leroy Jethro Gibbs.

Eleven years later, Harmon shrugs off any notions of feeling pressure at the center of that billion-dollar franchise. “Yeah, I don’t hear that,” he says. “I just think everybody does this together on this show, and hopefully that’s a similar footprint that will go onto NOLA. I don’t know how long this will last, I just know that the writers are still invigorated and the actors still like each other. Most of them don’t last a long time. Will we do this forever? No. But will we enjoy it while it’s here? Yeah.”

When he does wield his power, he does so sparingly, like making his displeasure known in 2009 when CBS first decided to go ahead with the show’s first spinoff, NCIS: Los Angeles, without his involvement. “I just had nothing to do with it,” he says now, when asked about his earlier resistance to NCIS spinoffs. So it’s no accident that Harmon himself is in the driver’s seat for NCIS: New Orleans, starring Scott Bakula, which sprung from his summer 2013 conversation with NCIS showrunner Gary Glasberg, in which he suggested that Glasberg’s proposed New Orleans storyline was worthy of its own show. “It really just came out of an idea, and talking about the coming season, which normally he doesn’t do to any great length with me,” says Harmon. “It just sounded like it was a series and not a two-episode arc.”

Harmon is credited as executive producer on New Orleans (he holds the same title on NCIS), but says his behind-the-scenes work isn’t just a way to keep things fresh after 12 seasons on the job.

“I wish I could blame it on that. It’s just working with Gary closely and trusting him and liking him,” says Harmon, who pushed to make New Orleans an integral part of the spinoff, so much so that the series films on location there, as opposed to the D.C.-based NCIS, which films in Los Angeles. “The sound, the color, the texture…there’s just something very important about shooting there.”

Though even Harmon admits he wouldn’t have been receptive to relocating to Louisiana for the job. “When you’re talking to actors, not only are you trying to attract them to roles that they want to play, but then on top of that you say, ‘You’ve gotta move!’ If someone came to me and said that in the beginning of this whole thing: ‘Hey, there’s this part we want you to play and be a part of it, but oh yeah, you’ve got to move to New Orleans,’ that would have been a different conversation. Because that’s not really the reason that I decided to do this.”

While most stars of long-running series, especially procedurals, would have long since moved on (think of the turnover on shows with epic runs like CSI, Law & Order, and ER), Harmon says he has no such temptations in Season 12. “I’ve done this for a while. Not this show, that’s not what I mean, but you get to a point where you learn to appreciate the ones where you work with people you really like, and that’s developed over the years,” he says. “And on top of it, it’s a hugely successful, highly-rated show. It might look easy, but it takes a lot to balance that over a number of years, and to try and do it well. And I think we do it well.

“I love this business and I’m trying to learn something new every day. It takes a partnership and it takes an experience like the show we’re currently doing, to realize that there’s so much to appreciate in this job. Sometimes you love getting up in the morning and sometimes you don’t. But for 12 years here, this group goes to work like that, which is nice.”

Still, if/when Harmon would ever change course and decide to move on, he insists that NCIS could go on without him: “I think this show can continue without anybody.” Glasberg, however, isn’t so sure: “It’s hard for me to say,” he told me earlier this year. “I guess that’s something that, over a glass of wine, we’ll figure out and decide what the future’s going to be—and if there is a future. A lot of people will have to chime in.”

But for now, it’s smooth sailing for (shh!) the world’s biggest TV star. “We had a bunch of bumps this past year, some of them we knew about, others we didn’t. And we survived,” Harmon says, referring to fan outrage over last season’s departure of Cote de Pablo, who played Ziva, after contract negotiations fell apart.

“I won’t be going there when it’s not fun, and I speak for a lot of people, it’s not just me. I just think that we is important on this show, not I. And because of that, a while back when this thing started to gather some momentum, one of the points I made to myself was to say, ‘Enjoy this. Appreciate it and don’t take any of it for granted.’ Which we don’t.”