When NCIS premiered in 2003, it was a modest spinoff of JAG, far from the global phenomenon it would become. In its early days, the series leaned more toward standard procedural storytelling than the personality-driven show fans know today.

Mark Harmon, who played Special Agent Leroy Jethro Gibbs for nearly two decades, revealed that the show was originally pitched as something very different. “When I first joined, part of what they sold me on was that this was all going to be based on real cases,” Harmon told The Hollywood Reporter. But the format quickly shifted to a “murd3r-a-week” structure – the familiar formula that would help the show climb ratings and eventually spawn five spinoffs, including the new prequel NCIS: Origins.

Harmon’s initial interest was in something more grounded – real Navy and Marine Corps investigations. He has since channeled that curiosity into nonfiction writing, co-authoring Ghosts of Honolulu and Ghosts of Panama with former NCIS advisor Leon Carroll Jr., exploring the true stories that inspired the agency.

In reality, NCIS agents handle everything from counterterr0rism and economic fraud to espionage and misconduct. As NCIS communications director MaryAnn Cummings told USO.org in 2016, “It’s the Hollywood version.” Real agents are more like detectives than action heroes, and there’s no equivalent to Abby Sciuto’s gothic lab or Ducky performing autopsies on the show. Still, the agency appreciates the attention, as the series raises public awareness of their work.

The show’s success didn’t come overnight. Harmon recalled that the first few seasons involved many adjustments before the formula clicked. By Season 3, the ensemble cast – balancing Gibbs’ stoicism with humor and camaraderie – gave the show a unique tone: part procedural, part workplace dramedy, all grounded in military grit.

While the “real-case” premise was largely abandoned, its influence remains through technical advisors like Carroll Jr., ensuring authenticity in dialogue, procedures, and the moral center of the series.

Had NCIS stuck strictly to real cases, it might have been more documentary-like and darker, potentially alienating viewers seeking entertainment and heroism. The suspension of disbelief, though, is what made the show a hit, offering larger-than-life characters and high-stakes mysteries week after week.

Now, with NCIS: Origins exploring Gibbs’ early years in 1990 – when the agency was still called NIS – Harmon is revisiting the grounded version of the agency that first captured his imagination. “It’s been historically interesting for me,” he said. “You’re learning about the agency in a different way than I had in the past.” After two decades, Harmon is finally exploring the show he originally envisioned – just in a way no one expected.