NCIS Fans May Be Shocked: Mark Harmon Admits He Never Knew This Key Detail About Gibbs, Says ‘Origins’ Co-Creator

Mark Harmon spent 19 seasons playing Supervisory Special Agent Leroy Jethro Gibbs on NCIS, so it’s understandable if a few details have slipped his mind. But according to NCIS: Origins co-creators David J. North and Gina Lucita Monreal, there’s one thing Harmon has forgotten that might shock fans.

The franchise’s first-ever “crossover through time” connects NCIS with its prequel, NCIS: Origins. Monreal explains that the episode explores a defining trait of Gibbs: his deep, sometimes obsessive commitment to victims. That theme ties directly to one of Gibbs’ most famous rules – No. 11, “When the job is done, walk away.” Harmon even recites it in a cameo at the end of the Origins half of the crossover.

North recalls mentioning the rule to Harmon during a recording session, only to discover something surprising. Fans constantly ask Harmon about Gibbs’ rules, but when North asked if he remembered any of them, Harmon replied, “There’s one about a knife.” The producers laugh at his forgetfulness, given how iconic the rules have become across the franchise.

Gibbs’ unofficial guidelines have been referenced since season 1, and by the time Harmon left in season 19, he’d delivered more than 40 rules that characters across all six NCIS spinoffs now quote.

The recent crossover begins on Origins with a deadly train accident that turns out to be a cr1me, leading to a wrongful conviction. Decades later, the present-day NCIS team gets a chance to finally solve the case when a prison break pushes it back into the spotlight. Although Gibbs left the series in 2021, his presence continues through weekly voiceover narration – and in this episode, through an on-screen appearance.

In the opener, Gibbs reflects on how time erodes details but not victims: “Back then, I thought maybe it would help to carry them with me.” That trait inspired the writers to show young Gibbs (played by Austin Stowell) cutting out victims’ photos for his notepad, making rule No. 11 central to the story.

Harmon’s real handwriting even returns; he personally wrote the rules used in the original series and the crossover. Monreal calls his penmanship “beautiful” and says he rewrote each rule many times for filming.

By the end of the episode, after the real culprit walks free, Gibbs admits he tried to follow his own rule to walk away – but says it’s “easier said than done.”

As for what’s next, NCIS star Wilmer Valderrama promises more surprises ahead: “There’s gonna be some major and nostalgic surprises, and we have a lot of big episodes.”