“NCIS” Star Brian Dietzen Recalls 'Meaningful' Final Chat with David McCallum, Crafting Show's 'Celebration' Tribute (Exclusive)

Dietzen tells PEOPLE that he and co-writer Scott Williams wanted to "do justice in honoring a TV legend" and pay tribute to the late actor's memory "by telling a good story"

Ahead of David McCallum's death last September, his NCIS costar Brian Dietzen was able to say goodbye in a touching way.

In an exclusive conversation with PEOPLE ahead of the NCIS tribute episode to McCallum, airing Monday night, Dietzen, 46, reflects on the last conversation he had with the late actor a few days before his death.

"I'll say that I got to speak to him on his birthday, which was wonderful," he says. "That was on a Tuesday. He ended up passing away that weekend, so he was already in the hospital and obviously not doing very well."

Related: 'NCIS' Star David McCallum Dead at 90: 'A Scholar and a Gentleman'

Dietzen shares how McCallum's wife, Katherine, passed the phone to him in his hospital bed. He adds that The Great Escape star was "very coherent" and Dietzen was "very happy to hear it."

"He was, obviously, going through some health issues and whatnot, but he had this stretch of time in that last week where he got to chat with all of his family, his kids, and obviously, Katherine," he continues. "And I was lucky enough that when I called in, I got to actually talk to him."

Although he plans to keep the "contents of the call" just between them, Dietzen adds, "I will say I'm so glad that I got to speak to him one last time and I got to tell him how I felt. And so that was really meaningful."

Related: David McCallum's NCIS Costars, Ex Joan Collins and More Honor Late Actor: 'He Made Every Moment Count'

In September 2023, McCallum died "peacefully" of natural causes surrounded by friends at New York Presbyterian Hospital, CBS announced at the time.

After studying at the Royal Academy of Dramatic Arts and doing several years in repertory theater in the United Kingdom, the Scotland-born actor moved to America in 1961. He landed the role of Illya Kuryakin in The Man from U.N.C.L.E. opposite Robert Vaughn. He earned two Emmy and a Golden nomination for the role. After joining NCIS in 2003, McCallum became best known for playing Dr. Donald “Ducky” Mallard on the crime series.

NCIS will honor McCallum's legacy with a special episode on Monday. Dietzen, who plays the protégé of his character Ducky, was tasked with co-writing the tribute with executive producer Scott Williams.

Related: The Cast of NCIS: Where Are They Now?

In addition to wanting to "do justice in honoring a TV legend," Dietzen tells PEOPLE that he and Williams wanted to "honor his memory by telling a good story."

"I think everyone in their own way was mourning this loss," he continues. "And I think that Scott Williams and I both had to recognize that we didn't want this show to fall into, well, this is just going to be forty-two minutes of us mourning and crying. There has to be a celebration about this."

Dietzen explains that he wanted to convey that not only has the NCIS universe benefitted from McCallum, but also how the "artists and humans outside of the show" have become "better for having known this awesome person."

"So that was our goal for the vibe on set was to say, of course we're saddened by this, and in addition to that, boy, we're going to celebrate the fact that we got to know this guy we got to work with," he adds.

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NCIS airs Mondays at 9 p.m. ET on CBS.

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