Chris O'Donnell and LL Cool J

It's ice-cream-sundae Thursday on the set of NCIS: Los Angeles, and almost everyone is indulging between takes. Even LL Cool J, who moonlights as a fitness guru. "Hey, you've earned it," a crew member tells the überbuff rapper/action hero, as if he needs anyone's consent for a sugar rush. The collegial atmosphere continues as Chris O'Donnell passes Daniela Ruah and Eric Christian Olsen, who wrapped their roles a half hour ago yet have stuck around to yak over their Ben & Jerry's. "Don't you two have homes?" O'Donnell demands to know, breezing on by before Ruah has a chance to kickbox his behind.

Apparently the lone holdout, O'Donnell hasn't taken any Cherry Garcia to his trailer. But once inside, he nods toward one thing he has allowed himself to indulge in on the Paramount lot: a sense of home. "I finally got some pictures of the kids up," he says, quietly chuckling at his reluctance to accessorize the space. "For Season 1, I was like, I'm not putting anything in my trailer. But now, I thought, I'll put some family pictures up. That's a Season 2 thing."

It's no wonder O'Donnell feels like he might be holding onto that trailer for a while: His show is firmly established as the second most popular scripted series on TV, trailing only NCIS itself, which precedes the spin-off by an hour on CBS. Like the mothership, the newer show has been enjoying a ratings uptick midway through the season. One February episode brought in more than 18 million viewers, the show's biggest tally since the premiere in 2009. It's clear that NCIS: Los Angeles and its audience are into a budding buddy relationship.

The surge is happening at a time when this ostensible ensemble series has cemented the fact that it's really about pairings. There has always been the show's A team, of course, with LL Cool J and O'Donnell as Sam and Callen, classic brothers-in-arms — and tight T-shirts. But thanks to some second-season cast changes, pretty much everyone else except mother hen Hetty (Linda Hunt) has been partnered up, too. Out in the investigative field, Ruah's Kensi and Olsen's Deeks are the B couple — although they earn an A for sexual chemistry. Back at the ranch, the C pair is Barrett Foa and newcomer Renée Felice Smith as cute surveillance geeks Eric and Nell.

As on the original NCIS, maybe only about 10 percent of any episode is given over to pure, plot-free characterization, mostly in the opening and closing bullpen scenes. But it's that 10 percent that hooks the audience just as much as the stunts and explosions and gunplay and saving-the-world stuff. "Chris and I try to understand each other's worlds, and the difference is part of the fun," says LL Cool J. "A kid from Queens and a kid from the north side of Chicago are probably not gonna meet, unless it's on Facebook or something. Those roads just don't cross. But this shows you how cultures can merge and how people can meet and connect."

There is a certain comfort-food aspect to seeing these very different characters get along so well. And while the spin-off is very much NCIS: Casual Dress, that informality plays against the fact that these agents rarely stoop to investigating mere dead sailors. They're much more likely to be saving the world, or at least Malibu, from terrorists. In the March 22 episode, Callen and Sam go undercover to prevent the assassination of a Venezuelan politician. As O'Donnell puts it, "In any real agent's career, what we do each week would be a highlight-of-their-career type thing. But hey, it's TV."

"Chris has that absolutely right," says exec producer Shane Brennan. "To spin off such a successful and much-loved show, it became apparent that it had to be familiar but different, so I decided to make NCIS: Los Angeles a show with a much stronger narrative drive. Stories move more quickly, and there's often a ticking clock, so by the time you hit that fourth act, there's a real imperative to stop something or save someone."    

For more with the cast of NCIS: Los Angeles, pick up this week's issue of TV Guide Magazine, on newsstands Thursday, March 17!

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