LONDON (AP) — Once again, Austin Stowell is having the best day ever — all thanks to him winning the role of legendary TV character Leroy Jethro Gibbs in “NCIS: Origins.”

“Since I got this job, it has just been day after day after day of the greatest day of my life,” says Stowell, smiling.

The actor has his shoulders back and chest up to portray the ex-Marine-turned-naval investigator, set 25 years before audiences first met “NCIS” star Mark Harmon.

Harmon and his son Sean are behind the idea of this origin story of the special agent, who was on-screen for 19 seasons from 2003 to 2021, solving crimes for the Naval Criminal Investigative Service in Virginia.

Stowell says he’ll be doing his best to live up to the role Harmon made famous and give viewers a new perspective on “how the hero was born.”

Harmon, who narrates and pops up occasionally in the show, has been very supportive of Stowell, making himself available to chat about life, visiting the set and even texting (something technophobic Gibbs would never).

“Mark and I talk a lot about what it means to be the leader of a team, about what it means to be a leader of this set and crew,” he says. “Those conversations have been invaluable to me because I don’t know what it’s like. I’ve never been No. 1 on a TV show before.”

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The lessons he’s learned: be on time, be kind, respectful and professional.

He’s also studied up on the “NCIS” universe, something he knew about but wasn’t yet a super fan.

In a pop quiz Stowell correctly names all the franchise’s four spin-off shows and only stumbles when it comes to rule three of Gibbs’ famous guidelines: “Never believe what you are told.”

(He keeps the full list to read from time to time.)

As for the enduring audience appeal of Gibbs, Stowell reckons it comes down to his humanity.

“Gibbs doesn’t wear a cape. He just has to use his brain and use his heart. I would argue that that makes him the most super of the heroes because it’s real. It’s something that we can all accomplish.”

“NCIS: Origins” isn’t just the procedural that people know and love, says Stowell, despite it having all the crime-solving and fun banter of the franchise.

“This is much more in the vein of a ‘True Detective’ or, you know, a darker crime piece. And that creates some, what could be uncomfortable situations on set. Very often I find myself kind of in a dark corner.”

His co-stars and fellow NIS investigators (the C hadn’t been added in 1991 when the show starts) include Mariel Molino as Lala Dominguez and Caleb Foote’s Randy.

It’s Gibbs’ first job since leaving the Marines. He’s got personal trauma and a big reputation, but he’s also got the sniper focus and built-in lie detector needed to be an integral part of this mystery solving team based at Camp Pendleton, headed up by Kyle Schmid’s charismatic Mike Franks.

“I just got to play this for the first time ... the other night where I look at a character and I just go, ‘You know, don’t you?’ And just get to bury them in my eyes,” Stowell says, laughing.

Those eyes have been enhanced by special contact lenses to provide the correct “Mark Harmon crystal blue.”

“NCIS: Origins,” which debuts Monday on CBS, has been shooting for three and half months. In that time Stowell has come to realize the parallels between himself and Gibbs, a character who mistrusts technology, loves nature and spends years building a boat in his basement.

When he got the call about getting the part, Stowell was off grid in Vermont.

“I’m very much an analog person, so I’m very comfortable in this 1991 world where the reliance is on conversations and relationships as opposed to Siri and Alexa.”

Has Stowell learned to trust his gut, Gibbs’ style?

“I read the pilot and immediately connected with who this guy was. And so my gut has told me that this is where I’ve been meant to be from the start,” he says, on the verge of tears.

“There is something that has awoken inside of me, almost like it was the character I’ve been waiting to play my whole life.”