It wasn't until Rocky Carroll's "NCIS" audition that he reconnected with Mark Harmon following their time together on "Chicago Hope." The actor joined during Season 5 and, as he told CBS, never expected to stick around on the crime procedural this long. He recalled how his audition went while speaking with CBS in May. He shared, "The next time I saw him was during the end of season five of 'NCIS.' My audition had less to do with me auditioning and more to do with [Harmon] saying, 'I know this guy, I like him, [and] you should hire him.'"
Carroll also noted his relationship with Harmon influenced the development of his character, NCIS Director Leon Vance, during the actor's time on the show. "Over time [Vance] becomes a part of the team, even though he is the head of the organization," he told CBS. "The writers gave me a lot of leeway with regard to that. Having had a relationship with [Harmon] prior to that was a huge help as well. We just have fun going back and forth."
In a 2009 HuffPost interview, Carroll said that Harmon truly is a "quarterback" and the "ultimate collaborator" given his ability to go back and forth with Carroll on screen. It's clear both actors have a level of appreciation for each other that ended up serving "NCIS" very well, which only makes Harmon's recent exit as a series regular on Season 19 all the more bittersweet.
Source : https://www.looper.com/641254/inside-mark-harmon-and-rocky-carrolls-friendship-on-ncis/269