NCIS Alum Mark Harmon Reveals His Favorite Of Gibbs' Rules — And He Follows It

Leroy Jethro Gibbs was practically a Boy Scout, so it makes sense that his "NCIS" portrayer Mark Harmon lives by the motto: Be prepared.

Harmon, who played Gibbs on the CBS procedural for 19 seasons, looked back on his days on the show when he joined current "NCIS" stars Brian Dietzen (Jimmy) and Diona Reasonover (Kasie) on the official podcast "NCIS: Partners & Probies" earlier this month. Appearing alongside longtime "NCIS" technical advisor Leon Carroll Jr., Harmon was asked by Dietzen which one of Gibbs' famous rules is his favorite — and Harmon was happy to demonstrate.

He reached into his pocket to pull out a Swiss army knife while admitting, "'Carry a knife' is a good one for me." (Rule #9, "Never go anywhere without a knife," was first introduced on "NCIS" way back in Season 1.) "You'd be surprised how useful it is," Harmon added, calling it "a handy tool" while adding a note of caution: "You can forget this sometimes when you're getting on an airplane, and then they take your little pocketknife, and then it's gone, which has happened, too."

Mark Harmon starred as head agent Leroy "Jethro" Gibbs when "NCIS" (a spin-off of the CBS military drama "JAG") debuted on CBS in 2003, leading a team that eventually included Michael Weatherly as Tony DiNozzo, Cote de Pablo as Ziva David, David McCallum as medical examiner Donald "Ducky" Mallard, and Pauley Perrette as forensic whiz Abby Scuito. (Current cast members Sean Murray and Brian Dietzen both debuted midway through Season 1 as McGee and Jimmy Palmer, respectively.)

Over the years, Gibbs rattled off dozens of his "rules" as words of wisdom to his younger colleagues, from Rule 10 ("Never get personally involved in a case") to Rule 51 ("Sometimes you're wrong"). Cast members came and went, but Harmon remained at the top of the "NCIS" call sheet as Gibbs until Season 19, when Gibbs said goodbye to the team to find peace and quiet in the Alaskan wilderness. Since then, Gary Cole's Alden Parker has assumed the role of team leader.

But Gibbs' presence was felt this past season when Gibbs' former colleague and friend Leon Vance (played by Rocky Carroll) was shot and killed in the line of duty, and it looked like the man responsible, corrupt CID director Wayne Rogers, was about to go free. Instead, Rogers was killed by a sniper's bullet — Gibbs is famously a great marksman — and Parker later cooked up some fresh Alaskan salmon, hinting that he and Gibbs pulled off a measure of revenge to honor their friend.

Watch the podcast in full here (skip to the 53:50 mark for the part about Gibbs' rules):