NCIS star David McCallum is loved for his role of Dr. Donald "Ducky" Mallard" on the CBS series, but the seasoned actor has held other famous roles as well. Now he can add "author" to his resume since his debut crime novel, Once a Crooked Man, is due to release next week.

POLL: How Will Tony DiNozzo Leave 'NCIS' At End Of Current Season?

With his novel about to hit bookstore shelves and NCIS still going strong the actor and author took to Reddit, a social networking community forum, for a question and answer session (AMA) with its users Tuesday.

Since his role as Ducky is so popular, McCallum spent time fielding a lot of questions about NCIS, which he has been included in since its inception. He actually appeared in its backdoor pilot on JAG in April 2003.

What Does Actress Emily Wickersham Think Of 'NCIS' Character Ziva David?

One topic that kept popping up was in regard to Cote de Pablo's departure from the series. She portrayed Ziva David and left at the beginning of Season 11. The majority of the time since de Pablo's wish to leave, the rest of the cast hasn't really discussed the situation, but Pauley Perrette (Abby Sciuto on the show) has defended Emily Wickersham (Eleanor "Ellie" Bishop on the show) since her character took over the female agent spot de Pablo's character previously held.

A Reddit user asked McCallum, "Any insight as to why Cote de Pablo left NCIS?"

"At the time I was astonished when Cote left," he admitted, adding, "But in retrospect, it seems that she was not up to handling the demands that the character put upon her. I still don't understand her deeper motives."

De Pablo came on the series after Sasha Alexander's character Special Agent Caitlin "Kate" Todd died in the Season 2 finale that aired in 2005.

Anyway, the actor shared other interesting tidbits of information in regard to the series, including reading Stedman's Medical Dictionary to know exactly what's being discussed in the many scripts.

"I learned early on that any dialogue that involves convoluted medical jargon, demands that I know exactly what I am talking about," he shared.

The actor also shared he has watched real-life autopsies happen so he "could learn the job first hand."

"The human body is miraculous," he noted.

Lastly, Ducky is known for going off on sharing stories frequently and it turns out McCallum "embellished" whenever there is a Scottish story.

NCIS airs on Tuesdays at 8 p.m. ET on CBS.

Tags: NCIS television CBS Cote de Pablo David McCallum