Mark Hamon and Michael Weatherly’s off-script moment in season one of NCIS became a staple throughout the show.

During episode five in season one, titled The Curse, of the CBS police procedural, Mark who plays Special Agent Leroy Jethro Gibbs slaps Michael’s character, Special Agent Anthony “Tony” DiNozzo, on the back of his head. That moment was improvised and was received so positively by the fans that it became a signature move for Leroy – especially with his interactions with Tony. The iconic move became dubbed the “Gibbs slap.”

In an interview with French publication Premiere (per Showbiz CheatSheet), Mark explained how the improvised moment came to be. The actor said: “We were doing a scene. And [Michael Weatherly] was on a Navy ship and he was talking to a female petty officer. I think this was in year one, early. And he was doing what he does, which is sometimes stay on script and sometimes not. I just reached over and smacked him. I tried to put him back online. It was an instinct. It wasn’t thought, I didn’t think about it, I just did it.”

“To his credit, he stayed in the scene; he didn’t break. And I didn’t break. And the girl in the scene playing the petty officer, she was shocked, she was surprised. And she stayed there, and we just kept going. And people liked it. And every episode I was smacking him–maybe too much. We still do that occasionally, but sparingly. That’s where it came from,” Mark continued.

The two worked together for 13 seasons from 2003 to 2016 until when Michael left the show. Mark and Michael did not have the greatest start in the beginning, and it’s been rumored the two were feuding. In a chat with Futon Critic in 2007, Michael spoke about how they were not close and did not mesh.

“Mark Harmon and I were on different poles. I was Antarctica, he was the North Pole and we were just looking down there going, ‘What is up with your polarity? What’s going on? The water goes down the drain the other way for me,’” the Dark Angel alum said.

Michael continued, bringing up an example of their earlier dynamic: “And he was confused by my presence right from the get-go. When CNN Showbiz Today or one of those things asked what was the best part about doing the pilot when we were doing the upfronts, I said, ‘I got a rubber gun and I got a fake badge.’ And Harmon looked at me like, ‘that is your CNN answer?’”

In a interview with USA Today (via Looper), Michael noted that being the lead of CBS’ Bull for six years from 2016 to 2022 offered him a new perspective and understanding to Mark.

“It’s been very illuminating and given me a great deal of insight and regard for what Mark Harmon was doing all those years. When I was like, ‘How come he doesn’t have as much fun as the rest of us?’” he elaborated.