Harmon, 72, left the long-running CBS drama in 2022 after 19 seasons.
Mark Harmon is sharing behind-the-scenes secrets about his time on NCIS and how he may not have starred in it if things -- including his character's name -- had been slightly different.
Harmon, 72, recently sat down with ET's Kevin Frazier to promote his new book, Ghosts of Honolulu: A Japanese Spy, A Japanese American Spy Hunter, and the Untold Story of Pearl Harbor.
The actor discussed the novel before the conversation turned to NCIS, the long-running CBS drama in which he starred as Leroy Jethro Gibbs for 19 seasons.
His participation in the show, however, may not have happened for a number of reasons including where he was at in his personal life when the script and auditions first came around.
"I didn't expect to like the script as much as I did when I first read it," Harmon said. "I was reading other things and I was also trying to stay home -- young family and I wanted to try and be home more."
What hooked the actor? It was the character's name, he shared.
"I read 'Leroy Jethro Gibbs' and thought, 'Huh, I like that name,'" Harmon said of what initially piqued his interest. "And then for a brief second when I decided that I liked the idea of the project, the name changed."
The name that Gibbs almost had was far less interesting and certainly a dealbreaker for the veteran actor who had come off of projects like Freaky Friday, The West Wing and JAG before NCIS began airing.
"Bob Johnson or something like that. And I went, 'No, no, it's gotta be Leroy Jethro Gibbs.' The creator said, 'No, you can't play a guy named Leroy Jethro Gibbs,' and I said, 'Why not?'" the actor and writer continued. "And then it went back and I was happy about it."
He also confirmed that it was not the network but the creator -- Donald P. Bellisario -- who wanted to change the name.
Harmon left the show in 2022 after 19 seasons to the shock and sadness of fans. He told ET he is still grateful for every episode and all of the success the show has seen.
"As an actor, you don't think in those kinds of terms," Harmon said in response to being asked about the series' 20th anniversary. "You're thinking, 'TV series, if it does three years, we're gifted.' But they've done well and they've worked hard and so it's a really good group of people."
"I don't know that any of us thought that the show was going to be around as long as it's been around," he continued.
The series went on to become so popular that it spawned numerous spinoffs including NCIS: Los Angeles, NCIS: Hawaii and the latest to join the bunch, NCIS: Sydney.
"We talked about this a lot ... over the years and I always thought that this show had characters, and it had humor, which made it different," Harmon said. "It had a case, but the case isn't what drove it -- I think that's still true."
Amid its monumental anniversary, Harmon -- who appeared in more than 430 episodes -- said he still understands why viewers come back week after week.
As for whether or not he would make a return to the show, he's not completely ruling out the possibility that viewers will see Gibbs again.
"He's probably sitting in a stream up in Alaska fishing," Harmon joked. "Is he going to get out of the stream? I don't know. But if he is, I don't know about it."
Harmon's book, Ghosts of Honolulu: A Japanese Spy, A Japanese American Spy Hunter, and the Untold Story of Pearl Harbor, is roughly about the start of the real-life Naval Criminal Investigative Service.
"When I first got the role in the show, I tried to google NCIS to figure out what it was -- I never heard of it -- there wasn't much information. And if you google it now, there's like 25 pages of information," he said, describing why it was important for him and Leon Carroll -- a longtime NCIS technical adviser -- to write it.
According to the book's description, it tells the tale of Douglas Wada, the only Japanese American agent in naval intelligence, and Takeo Yoshikawa, a Japanese spy sent to Pearl Harbor to gather information.
Ghosts of Honolulu: A Japanese Spy, A Japanese American Spy Hunter, and the Untold Story of Pearl Harbor is available now wherever books are sold.
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