NCIS producer and star Mark Harmon knew he'd found his young Leroy Jethro Gibbs the first moment he laid eyes on him.

When Bridge of Spies star Austin Stowell walked in to audition for the prequel series NCIS: Origins, Harmon recalled that he "felt the room jump a little," even "before he said anything." Harmon recently told PEOPLE about the search for the younger version of the character he played on the flagship series for 19 seasons. “There was heavy competition there,” Harmon recalled. “There were a lot of good people. I would say that there were really few who drew the attention Austin did.”

Harmon even remembered that Stowell "walked out and someone said, 'I'll just say it, that's a movie star.'"

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Harmon played Leroy Jethro Gibbs, a former Marine Corps sniper turned special agent and team commander for the Naval Criminal Investigative Service (NCIS), from 2003 to 2021. The long-running CBS procedural has already produced several spinoffs of varying degrees of success over the years, from NCIS: New Orleans, which Harmon co-produces, to NCIS: Los Angeles, NCIS: Sydney, and the recently canceled NCIS: Hawaii.

A spinoff exploring the events that led Gibbs to his post at NCIS was announced in January, with Harmon on board to co-produce. After extensive speculation over who should play young Gibbs (exempting Sean Harmon, the actor's son, who previously played a younger version of Gibbs on NCIS but returned to Origins solely as an executive producer), Stowell was announced as the official pick in March.

Discussing the kismet of Stowell's casting, Harmon remarked, "I can say that about any role that was cast in this show, because they all came in one after another. We got everybody we wanted." In addition to the Gibbs character, NCIS: Origins reprises the character of Mike Franks, the Major Case Response Team leader and mentor figure to Gibbs.

Muse Watson played the elder Franks on the flagship series, while Kyle Schmid (Being Human, Big Sky) was cast as Origins' younger counterpart.

Speaking to Entertainment Weekly in September, NCIS writer and executive producer David J. North said the Origins team "just knew when Austin came in that he was Gibbs... He embodied it. His talent is limitless — his dedication to the role, studying Mark, studying the show — he has a similar leadership quality that Mark had on set that bleeds into Gibbs the character."

Greg Gayne/CBS

"'You always have all of me,'" the actor remembered telling North and co-writer and executive producer Gina Lucita Monreal. "I just want to give everything to this. I know what it means to the fans. I know what Gibbs represents to a whole lot of people out there. I know what the show represents to a whole lot of people out there, and I don't want to let them down."

As an extra vote of confidence in the new series, Harmon himself will step from behind the camera back into the spotlight on tonight's series premiere. Harmon is set to make occasional appearances as the older Gibbs, narrating the past experiences that led him to working with the NCIS, which play out on Origins.

NCIS: Origins premieres Monday, Oct. 14 at 9 p.m. ET/PT on CBS, and streaming on Paramount+.