Mark Harmon weighs in on casting for 'NCIS: Origins' spinoff.

Mark Harmon possibly ended NCIS back to the fourth episode of season 19, but his connection to all things NCIS it's still going well. In keeping with this, he will publish the second of two books honoring the work of early NIS agents –Ghosts of Panama: A strong man out of control– on November 19, which he co-authored with him Leon Carroll Jr., a technical advisor for NCIS and a former one NIS Agent. And then he serves as executive producer and narrator of the prequel series NCIS: Origins, which just received its full season pickup.

“That was fun,” Harmon says parade taking over the narrative, “because both [showrunners] Gina [Lucita Monreal] And David [J. North] I have a long history with the series and have been involved with both for a long time. Since I always wanted to be there as a speaker, this was no surprise. What's the surprise when talking about Gibbs? Sometimes I don't have a lot to do in an episode, and sometimes I have a whole lot of things to do, and that means putting words in a character's mouth that traditionally say a lot with very little.”

When Origins was developed, it was always planned that Harmon would take over the narration, but what wasn't expected was that he would appear on camera in the premiere episode to explain the reason for the narration – and it was a pleasant reveal for fans , seeing that Gibbs is still there fishing in present-day Alaska and working on a diary.

“They were trying to figure out how to introduce the narrative, and that's why it was born,” Harmon continues.

Related: NCIS: Origins Star Austin Stowell reveals the personal tragedy he used to get into Gibbs' head

The reveal of what Gibbs is doing in 2024 was one of many Easter eggs that were part of the introduction of Origins. Others include Gibbs taking his first sip of bourbon, the first time he stopped an elevator for a private conversation, the fact that he always carried a knife, and other information about how Gibbs' rules came to be.

“The intention was always to have a lot of fun with it, and there are so many of those,” agrees Harmon. “David and Gina enjoy opening these.”

Harmon adds that he is just as much a part of it origins, The role is now Austin Stowell's and Harmon are available and will always be there when his help is needed, but now it's on to another era in Gibbs history that he's barely involved in. Except, of course, that he helped with the casting.

Related: Mark Harmon on Why Austin Stowell is the Perfect Young Leroy Jethro Gibbs NCIS: Origins

“We’re happy with the cast,” says Harmon. “Austin is a good boy and a nice actor. I think back to all the weeks we spent in casting offices putting this cast together and we couldn't be happier with this cast, but at the same time I think back to the original show, let's say the first four years, and the design of the show and how many changes there were in the first four years. There have been big changes and you assume that it won't be any different, that there will be changes.”

The good news is the fact that now NCIS: Origins After the series receives a full season with five additional episodes for a total of 18 seasons, it will have time to resolve all the problems.

NCIS: Origins airs Monday nights at 10pm ET/PT on CBS.

Related: NCIS: Origins Showrunner on how Mark Harmon's narration takes us inside Gibbs' head