Mark Harmon may have gone out NCIS back on the fourth episode of season 19, but its connection to all things NCIS still going strong. In line with this, he will release the second of two books, celebrating the work of early NOS agents –Ghosts of Panama: a strong man out of control– on November 19, which he co-authored with Leon Carroll Jr., a technical advisor NCIS and a former NOS intermediary. And then he serves as executive producer and narrator of the prequel series NCIS: Origins, who just received his full-season pickup.

“That was fun,” says Harmon Parade of taking over the story, “because both (showrunners) Gina (Lucita Monreal) And David (J. North) have a long history with the show and I go way back with both of them. So telling the story was always going to be part of this, so that wasn’t a surprise. What’s the surprise about Gibbs. Sometimes I don’t have a lot of things to do in an episode and sometimes I have a lot of things to do and that’s putting words in the mouth of a character who traditionally said a lot with very little.

When Origin developed, it was always intended that Harmon would do the story, but what wasn’t expected was that he would appear on camera in the premiere episode to establish the reason for the story – and it was a happy reveal for fans to see that Gibbs is still fishing in Alaska today and working on a diary.

“They were trying to figure out how to introduce the story and that’s what created the story,” Harmon continues.

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

The reveal of what Gibbs is doing in 2024 was one of many Easter eggs included in the launch of Origin. Others include Gibbs getting his first sip of bourbon, the first time he stopped an elevator for a private conversation, the fact that he always carries a knife, and more information about how Gibbs’ rules came about.

“The intention was always to have fun with that, and there are so many of them,” Harmon agrees. “David and Gina enjoy opening them.”

Harmon adds that he is part of it too Origin, the role is now Austin Stowell‘s and Harmon’s are available and will be there when his help is needed, but it’s now about a different era in the Gibbs story, in which he plays little role. Except, of course, he helped cast it.

Related: Mark Harmon on why Austin Stowell is the perfect young Leroy Jethro Gibbs NCIS: Origins

“We are happy with the cast,” Harmon said. “Austin is a good kid and a nice actor. I think back to all the weeks we spent in the casting offices putting that cast together and we couldn’t be happier with this cast, but at the same time I think back to the original show, like the first four years, and shaping the show and how many changes there were in those first four years. There were enormous changes, and you expect that there will be no different changes in this area.”

The good news is the fact that it does now NCIS: Origins has received a full-season pickup and five additional episodes have been added for a season total of 18, the series has time to work out all the kinks.

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

Related: NCIS: Origins Showrunners on how Mark Harmon’s narration takes us into Gibbs’ head