Mark Harmon and Michael Weatherly’s off-script moment in NCIS Season 1 became a defining feature of the show’s dynamic. During Episode 5, titled The Curse, Harmon, who plays Special Agent Leroy Jethro Gibbs, delivered an unscripted slap to the back of Michael Weatherly’s character, Special Agent Anthony “Tony” DiNozzo. This improvised gesture, later dubbed the “Gibbs slap,” resonated so strongly with fans that it evolved into a hallmark of Gibbs’ interactions with Tony throughout the series.

In an interview with the French publication Premiere (as reported by Showbiz CheatSheet), Harmon recalled how the moment originated. “We were doing a scene, and [Michael Weatherly] was on a Navy ship talking to a female petty officer. This was early in Season 1. He was doing what he does—sometimes sticking to the script, sometimes not. I just reached over and smacked him. It was instinctive. I didn’t think about it, I just did it,” Harmon explained.

Harmon continued, “To his credit, he stayed in the scene and didn’t break. Neither did I. The actress playing the petty officer was shocked but stayed in character. We kept going, and people liked it. After that, I was smacking him in almost every episode—maybe too much. We still do it occasionally, but sparingly. That’s how it started.”

Harmon and Weatherly worked together for 13 seasons, from 2003 to 2016, until Weatherly departed the series. Despite their on-screen chemistry, their off-screen relationship got off to a rocky start, with rumors of tension between the two. In a 2007 interview with Futon Critic, Weatherly described their early dynamic.

“Mark Harmon and I were on different poles,” Weatherly said. “I was Antarctica; he was the North Pole. We were just looking at each other like, ‘What’s up with your polarity?’ The water goes down the drain differently for me.”

He elaborated with an anecdote: “When we were doing the upfronts for the pilot, CNN Showbiz Today asked me what the best part of filming was. I said, ‘I got a rubber gun and a fake badge.’ Harmon looked at me like, ‘That’s your answer for CNN?’ He was confused by my presence from the start.”

Years later, Weatherly reflected on their time together and the pressures of leading a show. In an interview with USA Today (via Looper), he acknowledged gaining a deeper appreciation for Harmon after leading CBS’ Bull for six seasons from 2016 to 2022.

“It’s been very illuminating and has given me a great deal of insight and regard for what Mark Harmon was doing all those years,” Weatherly said. “Back then, I’d wonder, ‘Why doesn’t he have as much fun as the rest of us?’ Now I understand.”