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Finding an actor to play the younger version of Leroy Jethro Gibbs was the supreme "NCIS: Origins" challenge.

Mark Harmon embodied the character for 18 seasons of the original "NCIS," America's most-watched series for much of its 21-season run.

Finding Austin Stowell to play young Gibbs was just the beginning for executive producers Gina Lucita Monreal and David North, who had to flesh out a cast of new — and returning — characters stationed at NIS Camp Pendelton in 1991 for CBS' prequel series, set 12 years before "NCIS."

With input from Harmon, who narrates and executive produces the series, "Young Sheldon"-style, the duo had to find a younger version of Gibbs' rugged mentor Michael Franks, the Pendleton alum memorably played by Muse Watson.

"It's an arduous and stressful process because we know how much these roles mean to the fan base, and in the case of Gibbs, really in pop culture," says North.

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The final returning character involved NCIS Special Agent Vera Strickland, Franks' onetime partner, who was played by Roma Maffia and appeared in only one episode, but is a prequel-series regular.

Here's how the then-and-now casting went down for "Origins" (premiering Oct. 14, 9 EDT/PDT, then Mondays at 10).

"NCIS" star Mark HarmonHow he feels about 'NCIS: Origins' and new Gibbs, Austin Stowell

Diany Rodriguez will light up Vera Strickland's 'Origins' world

Longtime "NCIS" producer and writer Monreal pounced on the Season 11 appearance of Maffia's Special Agent Strickland. "It was a great pull," says North, "because Strickland is turning into a great 'Origins' character."

Monreal says Maffia created a full character in her solo "NCIS" episode, which "Origins" explores with NBC's "The Blacklist" star Diany Rodriguez, who appeared in a 2017 episode of "NCIS: New Orleans" as DHS Agent Garcia.

"The essence of Strickland was already there, but Diany is bringing her own thing," says Monreal. "This character can go in many ways, and we're taking her there."

Strickland, along with new character Cecilia ‘Lala’ Dominguez (Mariel Molino), will explore the male-dominated world of NIS Pendleton three decades ago.

"There weren't many female agents in 1991, so Lala and Vera hold down the fort," says Monreal. "We'll see how they navigate what is really a man's world."

Kyle Schmid brings the intensity, and the facial hair, to Mike Franks

Franks, the gung-ho former Marine, was such a critical part of "NCIS" that Gibbs' mentor continued to appear in flashback episodes even after being murdered in Season 8. "Origins" found the power ― and the facial hair ― with Canadian actor Schmid.

"There was the physical likeness, and there was that energy that Kyle brings," says Monreal.

Most significant was the chemistry between Schmid, 40, and Stowell, 39, who had already been cast as the young Gibbs. "After seeing that exchange, I walked out of the room and said, 'That was Franks and Gibbs right there.'" says North.

Austin Stowell buzzes into 'NCIS: Origins' as young Gibbs

Stowell earned the instant Gibbs endorsement from Harmon and his son, "Origins" executive producer Sean Harmon, who often played the young Gibbs in "NCIS" flashbacks.

"After seeing Austin in person, it was like, 'Oh my God, this is Gibbs," says North. "When we called Austin to let him know he had the part, he was in a cabin in the woods and almost missed our call because of no internet. So young Gibbs, in the middle of nowhere."

To demonstrate the level of fan scrutiny, the internet pounced on an early look of Stowell's Gibbs, also a former Marine, without the buzzcut seen in "NCIS" flashbacks. The NCIS-verse official TikTok account posted a video of Stowell getting his "Origins" cut, and later images have stuck with forever-shorn hair.

"There was some blowback from the first picture. It was simple as we weren't anywhere close to shooting," says North. "His hair is now Gibbs-appropriate."