Mark Harmon has been a successful actor for decades with a career dating to the early '70s and a 19-year stint on the massively popular show "NCIS." The TV star spoke to Fox News Digital about the success of the series while also promoting the release of his new book, "Ghosts of Honolulu," co-written with Leon Carroll Jr., a former NCIS special agent who also acted as an adviser on "NCIS." Harmon said the show is a big hit in part due to the camaraderie of the cast and crew. "I think you're really lucky to find that," he said while sitting next to Carroll. "At times it's really rare. I always thought that the show was about characters and it had humor. And yeah, there was a case and originally the case was based on the real. And then television changes, showrunners change, writers change, actors change, lots of changes. And that changes as well." ‘NCIS’ STAR MARK HARMON CREDITS MAKING ‘COLD CALL’ FOR 36-YEAR MARRIAGE TO ‘MORK & MINDY' STAR PAM DAWBER Considering his working relationship with Carroll, he said, "For us now, to take a 20-year relationship, a daily 20-year relationship, and now turn this back into telling the real story of the agency and talking to two agents and individuals, who never before have been asked to speak of what we're asking them about, has been a treat. And I've learned that. By working alongside him for the last 20 years, and that's about respect." Harmon said that when he was first presented the idea for "NCIS," based on the Naval Criminal Investigative Services, 20 years ago, he'd "never heard of it before. There wasn't much information to be found." Then Carroll came into his life. WATCH: Mark Harmon explains why he joined ‘NCIS’ "I immediately met Leon. I've been with him for 20 years, every day on the show," he said. "There was never an interrogation scene on that show that I didn't talk to him about. … I had so much respect for the job this man did in the real world, the real NCIS. And we always talked about that." "And there was a moment at some point in time fairly recently where this opportunity [to write the book] came forward. And I basically said I would not touch this without having Leon Carroll be part of it." "Ghosts of Honolulu" dissects recently uncovered documents related to Pearl Harbor in what publisher Harper Select calls "a true-life NCIS story of deception, discovery, and danger." It tells the story of Douglas Wada, "the only Japanese American agent in naval intelligence" at the time, and Takeo Yoshikawa, "a Japanese spy sent to Pearl Harbor to gather information on the U.S. fleet." While Harmon is busy promoting his new book, certain rumors have hinted that he could eventually make a return to "NCIS." Earlier this year, executive producer Steven D. Binder hinted in an interview with TV Insider that his character may make another appearance. Harmon left the series in 2021. "I don’t see how we don’t see him one more time at some point," Binder said. "Gibbs has, in my mind, advanced to a higher plane of existence for now. … We left him smiling on a river happy, and that’s the image I want people to have until we’re really ready to blow that out of the water or truly embrace it in some way." ‘NCIS: NEW ORLEANS' STAR LUCAS BLACK EXPLAINS WHY HE LEFT HIT SERIES: ‘ENOUGH WAS ENOUGH’ While fans are eager to see Harmon return, the actor revealed to Fox News Digital that the only reason he accepted the role in the first place was because of his character's unique name – he played Special Agent Leroy Jethro Gibbs from the show's premiere in 2003. "I was reading a lot of scripts, and I read this one, and the name Leroy Jethro Gibbs hit me," he recalled. "And there was a moment there where in that couple of days I think it changed to Bob Robinson or something. … And I right away called up and said, ‘The name has got to change back.’ And then someone said, 'You can't play a guy named Leroy Jethro Gibbs.' I said, ‘Why not?’" He confessed, "If that name had not been there, I don't think I would have been there." WATCH: Mark Harmon discusses marriage with Pam Dawber Even with Harmon's absence from the cast – he remained on the show as a producer – "NCIS" has remained hugely popular. The Wall Street Journal reports that "the original ‘NCIS’ averaged nearly 10 million viewers per episode last season, making it the most popular CBS show as well as the most-watched television drama on broadcast TV." The publication added that "NCIS' and its spin-offs," which include "NCIS: Los Angeles," "NCIS: New Orleans" and "NCIS: Hawaii," along with "NCIS: Sydney," which debuted last night, "have never been the darlings of critics and don’t clean up during awards seasons, overshadowed by streaming’s ‘prestige TV’ fare, but the franchise has generated about $8 billion since the first ‘NCIS’ launched, according to people familiar with the matter." LIKE WHAT YOU’RE READING? CLICK HERE FOR MORE ENTERTAINMENT NEWS While Harmon is undoubtedly successful in his career, he's also had a big win in his personal life. In 1987, he married "Mork & Mindy" actress Pam Dawber, and the two have been married ever since. Harmon told Fox News Digital that he's "fortunate" and "lucky" to be with his wife. When asked about the key to a successful marriage , the actor said, "I just consider myself fortunate … what a great lady, and I'm lucky." "I don't know," he admitted. "I don't have any answers. I don't have any key. There's no ready answer, you know." WATCH: Mark Harmon didn’t know what NCIS was when he joined the show Harmon and Dawber, both 72, met through a "cold call" he made to her after a friend suggested the pair should be introduced on a group date. CLICK HERE TO SIGN UP FOR THE ENTERTAINMENT NEWSLETTER "I said, 'Can I just call? Can I get a number and just cold call?' And so I did," Harmon told People magazine this month. He explained, "I got the number, and I called. And I got an answering machine. And I started to leave a message that said, ‘We don't have to [all] go out. We could get a cup of coffee or something.’ And then she was monitoring and she picked up. We went out that night, and we're together ever since." Exactly one year later, they were married. Harmon and Dawber share sons Sean, 35, and Ty, 31, who both work in the entertainment industry. Sean is an actor who appeared on "NCIS" and "NCIS: Los Angeles," and Ty is a screenwriter. Fox News Digital's Ashley Hume contributed to this report.