Finding a suitable actor to portray a younger Leroy Jethro Gibbs was the ultimate challenge for “NCIS: Origins.”

Mark Harmon portrayed the character for 18 seasons on the original “NCIS,” which has been the most-watched series in the U.S. for much of its 21-season history.

Securing Austin Stowell for the role of young Gibbs was just the beginning for producers Gina Lucita Monreal and David North, who had to build a cast of both new faces and familiar characters based at NIS Camp Pendleton in 1991 for CBS’s prequel series, which takes place 12 years before “NCIS.”

With guidance from Harmon, who serves as narrator and executive producer, the pair needed to find a younger version of Gibbs’ tough mentor, Michael Franks, who was famously portrayed by Muse Watson.

“It’s a challenging and intense process because we understand the significance of these roles to the fans and to pop culture, especially in Gibbs’ case,” North remarks.

The last returning character they sought was NCIS Special Agent Vera Strickland, who was portrayed by Roma Maffia and only appeared in one episode but will now be a regular in the prequel series.

This is how the casting process for “Origins” unfolded (set to premiere on Oct. 14 at 9 EDT/PDT, continuing Mondays at 10).

Diany Rodriguez brings vibrancy to Vera Strickland’s ‘Origins’ story

Longtime producer and writer Monreal seized the opportunity after Maffia’s portrayal of Special Agent Strickland in Season 11. “It was an excellent pick,” North says, “as Strickland is evolving into a significant character for ‘Origins.’

According to Monreal, Maffia developed a well-rounded character in her standalone “NCIS” episode, which “Origins” aims to expand upon with Diany Rodriguez, who previously appeared in a 2017 episode of “NCIS: New Orleans” as DHS Agent Garcia.

“The core of Strickland was already evident, but Diany adds her unique touch,” Monreal explains. “This character has various paths she can take, and we’re excited to explore those.”

Strickland, along with new character Cecilia ‘Lala’ Dominguez (Mariel Molino), will delve into the male-dominated environment of NIS Pendleton from thirty years ago.

“In 1991, there weren’t many female agents, so Lala and Vera are really holding their ground,” Monreal notes. “We’ll witness how they navigate a world that is predominantly male.”

Kyle Schmid adds depth and character to Mike Franks

Franks, the enthusiastic former Marine, played a vital role in “NCIS,” continuing to appear in flashbacks even after his character’s death in Season 8. “Origins” successfully captured the energy and essence with Kyle Schmid, a Canadian actor.

“He had the physical resemblance, along with the dynamic energy that Kyle provides,” Monreal notes.

What stood out most was the chemistry between Schmid, 40, and Stowell, 39, who was already chosen for the role of young Gibbs. “After witnessing their interaction, I left the room and thought, ‘That was Franks and Gibbs right there,'” North shares.

Austin Stowell shines as young Gibbs in ‘NCIS: Origins’

Stowell received immediate approval from Harmon and his son, Sean Harmon, who frequently portrayed young Gibbs in “NCIS” flashbacks.

“After seeing Austin in person, I thought, ‘Wow, this truly is Gibbs,” North remarks. “When we called Austin to inform him he got the role, he was in a remote cabin and nearly missed our call due to lack of internet. So, there he was, young Gibbs, in the wilderness.”

Quickly, the fanbase took note of Stowell’s early appearance as Gibbs, a former Marine, before the buzz cut style seen in “NCIS” flashbacks.

The official TikTok account for the NCIS-verse shared a video featuring Stowell getting his hairstyle changed for the “Origins” cut, and subsequent images show him with shorter hair.

“Some fans had a reaction to the first image. It was clear that we weren’t close to starting filming,” North explained. “Now his hairstyle is suitable for Gibbs.”