Mark Harmon reveals what it's really like to return to NCIS

Three years after his departure, Mark Harmon is back on NCIS — sort of.

Tomorrow, Monday, October 14, NCIS fans will finally get to see NCIS: Origins , which tells the origin story of the West Wing actor's legendary character Special Agent Leroy Jethro Gibbs, which he played for a whopping 18 years.

And though he won't be himself starring in the prequel show, instead Austin Stowell will star as a young Gibbs in 1991, he is a producer and narrator of the show, and is eager to lend his support, and see his character's backstory get told.

Speaking with CBS about the franchise's latest installment, Mark shared: "This is a chance to really kind of dig into it."

Fans will get to see a more personal, and emotional, side to Gibbs, as the plot explores the aftermath of him losing his wife and their child. "He's in rough shape," he noted.

Austin, who has previously starred in The Hating Game, The Inheritance , and Keep Breathing , also opened up about what it has been like to fill Mark's shoes.

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"You come in and audition here for years and years, and all of a sudden, you're presented with a badge with your name on it," he first said of working on the iconic Paramount lot.

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He then confessed: "I felt very confident in what I could bring to the character, and then the second you walk in the room, that all goes out the window when you're staring at the guy who has done it for 20 years."

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But Mark is confident in his choice. Recalling his casting, he shared: "I thought Austin had a presence. I think Austin walked out and you felt the room jump a little bit."

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Moreover, Austin has of course had not only Mark's approval, but his support, every step of the way.

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"From day one, Mark has been available," he said, adding: "He's so good at allowing the people who are on this show to feel like they are supported."

Mark then noted: "I'm there to help and to talk to them or to tell them what I remember from being in this for a while," though emphasized:  "But this is their thing."