Three (hundred!) cheers for NCIS!

The long-running CBS drama is hitting a huge milestone this year with its 300th episode, and ET is bringing you the inside scoop straight from Mark Harmon, Michael Weatherly, and the rest of the all-star cast at Tuesday's on-set celebration.

So what's the secret to their success? "You kick it right back to the scripts and people," Harmon spilled with a smile. "It's a very different collection of professionals who all are proud of what they do. They work hard every day, and we like coming to work, and we like each other."

MORE: Michael Weatherly Is Leaving 'NCIS' After 13 Seasons: 'It's Been a Fantastic Ride'

NCIS, which still reigns supreme as TV's most-watched drama, will air its 300th episode on Tuesday, March 15.

"I've said often, there's nothing wrong with loving your job, and there's a lot to love here," Harmon, who has brought Special Agent Gibbs to life since the beginning, continued. "We've worked hard to get it to this place and [now] it's about taking an appreciation of that."

Making the 300th celebration even more emotional was the fact that Weatherly will be leaving CBS drama at the end of this year. After 13 seasons as Special Agent Tony DiNozzo, the 47-year-old actor announced last month that he will be moving on.

"I have had the most extraordinary journey and ride," Weatherly spilled to ET. "It's been a hugely gratifying and awesome experience and I pray that when I'm in my old folks rest home -- which is only a few years from now -- that I will look back on these 300 episodes. I'm really most impressed with the audience that found this show and the fans out all around the world who love these characters. If anything, that's what's kept this show going."

When asked how the series has made it this far, the quick-witted star had a clever response. "It's about teamwork," Weatherly joked while his co-star Emily Wickersham rested on top of his shoulders. "Some of us have to carry the others a little bit." Weatherly also revealed to Wickersham his "biggest piece of advice" for her to follow after his departure: "Have fun!"

WATCH: Pauley Perrette Reveals How Different 'NCIS' Will Feel Without Michael Weatherly

Despite Weatherly's upcoming departure, NCIS' highly anticipated 300th episode will be jam-packed with the series' signature high stakes, and will also feature an array of top-notch guest stars.

"I'm the biggest NCIS fan! I think it's the greatest show ever, so I'm just stoked. I'm so excited and so proud," star Pauley Perrette gushed. "My favorite moment is every morning when I get up and I just thank God for my job. I'm just like,'Yes! I woke up again and I still get to play Abby Sciuto on NCIS each and every day!'"

The long-running series, which has spawned two successful spinoffs -- NCIS: LA and NCIS: New Orleans -- has attained global popularity over the years, and according to Brian Dietzen, there's still plenty more episodes to come.

"Everyone hopes that their show will obtain 300 episodes and around here, that was the big hope," Dietzen dished. "[But] not just 300 -- we're halfway there. We want the 600 mark with 26 years, and so after that we might slow down, or we might not. Who knows?"

Harmon added, "Look, they all end at some point, and who knowns when this one will end, but for now it's important to take time to enjoy this in the midst of everything that we're experiencing."

The 300th episode of NCIS will air on Tuesday, March 15 at 8 p.m. ET/PT on CBS.