NCIS , NCIS: Los Angeles, and NCIS: Hawai'i stars preview how the 3-show crossover is like the MCU

For a group of people being targeted by a mysterious and highly-trained hitman, the mood on the set of the first-ever NCIS three-show crossover is surprisingly light. Upbeat funk music is playing and NCIS: Los Angeles star LL Cool J is singing along, NCIS: Hawai'i's Vanessa Lachey is dancing, and NCIS' Wilmer Valderrama is talking about how he recently binge-watched House of Cards and was blown away by the acting. Standing in the NCIS: LA headquarters, Chris O'Donnell and Gary Cole are all smiles. But then someone calls out "action!" and the mood instantly changes. No one's smiling, dancing, or joking around, because it's time to get serious as the actors film the last hour of the three-episode crossover event, in which all five characters share the screen for the first time ever.

"This is the first time that we're in the bullpen and we're trying to break down what's going on, it's the first time with Gary — this is a big day," Lachey tells EW on the Los Angeles set in between takes. "We're doing something that has never been done before in the history of NCIS and that is bringing all three of these shows together. It would've been amazing if NOLA was still around and we could have had the four of us, a little quad, but it's just an honor to be a part of this now."

In the crossover event (check out EW's exclusive first-look photos from each episode below), all the fan-favorite agents from each show team up to solve a case in a heart-pounding story line, as they attempt to take down a mysterious threat who is targeting several of their own. The story begins when team members from the different NCIS divisions come together in Washington, D.C. to celebrate a beloved FLETC (Federal Law Enforcement Training Center) professor who taught multiple members of each squad. On the eve of the celebration, the professor is found dead of an apparent suicide, but his former students immediately suspect foul play. While investigating his past, they uncover a mysterious hitman, and the more they dig into it, the bigger the targets on their backs become. When members of the team are suddenly kidnapped, the agents assemble to take down the assassin and save their own.

Sonja Flemming/CBS (L-R) Katrina Law, Vanessa Lachey, Wilmer Valderrama, and Chris O'Donnell on the 'NCIS' crossover

In the scene they're filming on this early October day, Lachey's Agent Tennant takes control of the case after all the others try to assume authority. "I am the special agent in charge in Hawai'i, and I get to talk to Todd [LL Cool J] and Chris' characters and Gary's character and Wilmer's character and stand literally beside them, demand respect, and take over leadership," Lachey says. "She's a badass woman with a warm heart commanding all these strong men with grace and humor, and it's a really cool moment for me." She laughs before adding, "I love my job."

Watching all five leaders of their respective shows battle for authority is thrilling, as they're each larger than life on their own home turfs. But now, they're all together in a moment that fans have been dying to see for years — and the reality is even better than what anyone could have imagined. "It's a pretty traditional chain of command amongst each show office and that's being thrown together because people have different positions within NCIS and suddenly it all gets mixed up," O'Donnell explains. "It's interesting, at first it's like they're coming in our home and telling us what to do. 'Wait a second, Kilbride's not here,' but there's a chain of command so you have to respect that."

While this scene takes place in the third and final hour of the crossover, it's actually the first time all five actors have been in the same scene together, and they all keep talking about how cool it is to finally get here. "It's like all the superheroes of procedurals coming together for the first time in history. It's iconic," Valderrama marvels. "It's going to feel like it should've happened a long time ago, and I think it's going to feel like a tribute for the trajectory of all these shows, especially a really incredible moment to honor the mothership after 20 years of NCIS."

Sonja Flemming/CBS Gary Cole, Vanessa Lachey, Sean Murray, Wilmer Valderrama, Noah Mills, Katrina Law, and Brian Dietzen on the 'NCIS' crossover

LL Cool J was surprised at how easily all five actors got into the swing of things while filming this scene especially. "It felt like we've been doing that for years together, not as separate groups. That's pretty special," he says. "There'll be action. There'll be thrills. There'll be rollercoaster rides. All of the chills and the fun and the craziness that they are used to, but seeing how the personalities interact — it's like how Batman and Superman interact, or in the Marvel Universe, how all the different superheroes interact. Seeing these characters from the NCIS-verse come together and the way they communicate and connect is going to be interesting, and I think fans will be excited about that."

But while each actor can't stop praising the excitement of this moment, they all know why it's taken so long to see a three-show crossover come to fruition, despite how badly the fans have been asking for it. "It logistically is really challenging for the production. And it's tough on the writers," O'Donnell says. "I mean, not only do they have to make one story line that goes through three episodes on three different shows, they've got to keep their own show going and come up with a story line where you're missing some of your key actors. That's really a challenge. All the credit goes to the writers and the producers that are able to make it work."

Cole says with a laugh, "Whoever figured out the shooting schedule, they need to get a big gold star."

Karen Neal/CBS Vanessa Lachey, Jason Antoon, Tori Anderson, and Gary Cole on the 'NCIS' crossover

Bringing together all three shows for one event meant making the case they're solving bigger and more personal than anything they've encountered before. "There's somebody who's out for the heads of everybody on the team, and this is a challenge that just one particular squad or one team of NCIS wouldn't be able to handle," LL Cool J says. "It's a larger threat than we normally have to deal with, and geographically this threat spans multiple regions, so it takes the trifecta, it takes all three of us coming together to deal with it. And there's a really important NCIS member who's missing as well, so it gets crazy."

O'Donnell loves how the threat level is more intense than ever. "We realize we're up against some formidable foes, and we're suddenly the hunted," he teases. "This person has now put us up on this website, all our photos, and has every gang member and cartel member trying to hunt us down. We've got to watch our step here. Certainly if you're going to bring together all these shows, you might as well raise the stakes and ratchet up the action and the drama, the humor, everything."

Karen Neal/CBS Alex Tarrant and Chris O'Donnell on the 'NCIS' crossover

But when mixing three different shows, it became a challenge to make sure each show is represented equally. "I know the fans will be curious to see how these worlds collide, since this is something they've wanted to see for a long time," O'Donnell says.

"All three shows have their own distinctive styles," Cole explains. "They seem to carry a lot more heavy weaponry on this show than we ever get involved with, like grenade launchers and helicopters. We're all just carrying our little pistols around. And Hawai'i has great weather. The challenge is to get comfortable as quickly as you can. I really go moment by moment, scene by scene and just do your part of it, fit into the puzzle of that story."

Valderrama was especially excited to see how each show's tone was brought together into one story. "The tricky part is really making sure that tonally, we all meet in a line in what we got to do and that our characters are performing at their strength and where we were meant to be created at. And when you talk about all these alphas meeting, working together, it's about love and it's about respect for each other's environment. Every single one of the team members in the show are very confident within their characters so if the scene is written effortlessly, then everyone's going to have a beautiful moment to shine. And we've all known each other for years, so there's a level of familiarity where all of us are rooting for each other and making sure that we have what we need to shine."

Michael Yarish/CBS Wilmer Valderrama and Vanessa Lachey on the 'NCIS' crossover

Now that the first three-show crossover is about to air, the stars are looking forward to doing even more large events like this in the future. "The NCIS-verse, the fans always wanted it," LL Cool J says. "Since we've been doing the show, fans have always asked me, 'Are you ever going to be on the original?' So this was a natural step and a natural progression to me. It makes sense. It's fun, it's innovative. Working with everybody, that's what it was about for me, to be honest — working with new people and putting yourself in the mindset of a fan that is used to seeing these people in certain bubbles and places. And now they're piercing a veil and crossing into another world. They're getting to see how these characters interact and the energy around it is special. It feels like it's the beginning of something beautiful."

Cole is "sure" that fans will see more three-show crossovers in the future. "It's as natural a way to write these stories as anything because in reality, agencies that are nationwide and have regional offices have to work together because bad guys don't stay in one place all the time," he says. "I think that makes sense. And when you do [these crossovers], there always can be tension and conflict about who's in charge because in reality, there's been people rubbed the wrong way from different agencies that collide on each other and fight for control. But you can't demand respect, you have to earn it, and hopefully you see all these characters doing just that."

Originally scheduled to air Jan. 2, the three-show crossover will now premiere Monday, Jan. 9, beginning with NCIS at 8 p.m. ET/PT, continuing with NCIS: Hawai'i at 9 p.m. ET/PT, and concluding with NCIS: Los Angeles at 10 p.m. ET/PT on CBS.

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