On the NCIS: Los Angeles set this March day, there’s even more bull in the bullpen than usual. The number of buff leading men wearing too-tight shirts has suddenly doubled, and so has the quick-fire banter.

Several of the law enforcers on hand need to be inoculated against smallpox, since their mission is to bring down a madman intent on infecting the population with the virus. “I’m not a big fan of needles,” squeaks Eric Christian Olsen, as Deeks. “I’ll buy you ice cream if you don’t cry,” promises Kensi (Daniela Ruah). Linda Hunt, aka Hetty, plays the straight woman, telling Deeks: “You might feel a little prick.” “Probably not his first time,” adds Scott Caan, who plays Det. Danny Williams.

Say aloha to the enchanted land of crossover TV. The two-parter begins on Monday’s Hawaii Five-0 with Callen (Chris O’Donnell) and Sam (LL Cool J) heading to Hawaii to help track down the smallpox terrorist — who just may be someone from Callen’s past. The action continues the following night on NCIS: Los Angeles, when Danny and Chin Ho (Daniel Dae Kim) visit their Navy-investigator comrades on the mainland. “It’s an alliance,” says LL Cool J. “You have these two crack teams that have to stop this guy before he hurts innocent people. Sometimes the job is too big for one crew.”

NCIS: L.A. creator Shane Brennan can relate. “Crossovers are not easy to pull off when you’re working in the same town,” he says, “and when you’ve got the other show being shot a five-hour flight away, it’s particularly challenging.”

To make things more complicated, Part 2 of the crossover — the NCIS: L.A. episode — was not only shot first, but before the actors even had the Hawaii Five-0 script that sets up the story. “It was a little bit backward way of doing things,” says O’Donnell. Adds LL Cool J: “We shoot out of sequence so much anyway that it’s about imagination. That’s just part of what we do as actors. Sometimes we have to play an NBA championship with no basketball.”

The instant camaraderie between the casts helped make filming easier. “Daniel and Scott fit right in here,” says O’Donnell. “It is funny for me seeing unfamiliar faces in this room, because this is the first episode where anyone had special permission to come inside the ops center.” Guest stars haven’t previously been allowed any closer to the series’ super-secret, Spanish-tiled HQ than the boat shed, but then, other guest stars don’t draw 10 million viewers a week, which truly merits a special hall pass. “And for us, it’s strange to go to Hawaii and be playing the same characters, but the crew is completely different,” O’Donnell adds. “It’s kind of bizarro world.”

Caan agrees. “There are parts of doing this [crossover] that are weird,” he says. “There are writers on our show who have written 40-something episodes, so they know my thing or Daniel’s thing. And then we’re gonna tell the NCIS: L.A. writers, ‘Here, you give it a shot’? You can be skeptical about that. But I think it worked out really well. And when you’re working with the same people over and over again, it is fun to work with new people — and having them be LL Cool J and Chris just makes it better.”

One of those guys, in particular, made it better for Caan. “I don’t want to take anything away from Chris, but LL Cool J is definitely somebody I’ve been paying attention to for more than half my life,” says Caan. “I grew up listening to hip-hop music. We had a bunch of long conversations, and he’s a really smart cat. Within two weeks, he changed my mind about a lot of important things, and I’m pretty stubborn.” Caan doesn’t want to get into specifics, but he alludes to LL Cool J’s emphasizing a balance between perfectionism and just getting the job done that struck a chord with his work psyche. Says Caan: “He definitely has a leadership thing down.”

While Brennan admits the idea for the crossover episodes came from CBS brass, this seeming shotgun wedding didn’t actually take place at gunpoint. “There’s no edict from on high,” he says. “They said, ‘What do you think of the idea?’ And [Five-0 executive producer] Peter Lenkov and I said it was a great one.”

“For me as a musician, it’s like two bands going out on tour,” says LL Cool J on why he liked the idea. “You know, no reason not to do it. It’s just a better experience for the people who buy a ticket. And I don’t think it’s going to hurt either show.”

For more on the NCIS: Los Angeles/Hawaii Five-0 crossover, pick up this week’s issue of TV Guide Magazine, on newsstands Thursday, April 26!