From ‘JAG’ to ‘NCIS’ and now ‘NCIS: LA,’ CBS has found a letter-perfect formula

Between takes on the set of “NCIS: Los Angeles,’’ the actor looks relaxed, if a little gruffer and buffer than in his “Scent of a Woman’’ days, thanks to a close-cropped ’do and 5 o’clock scruff. But, he admits, he was a little concerned that CBS wasn’t actually picking up the “NCIS’’ spin-off when he saw how many hits he was taking in a two-part episode of the original series, which served as a de facto pilot for the new show.

“I was a little shocked,’’ he says with a chuckle, “because that was a lot of bullets.’’

O’Donnell lived. And if there is a bulletproof new series this fall TV season, it’s “NCIS: Los Angeles.’’

Premiering Tuesday at 9 p.m., the crime drama literally has a built-in audience since CBS has decided to air it directly after its mothership show. The two-parter, which attracted 16.7 million viewers, introduced the Office of Special Projects, an undercover unit of the Naval Criminal Investigative Service. The unit is led by O’Donnell’s dark and mysterious agent “G’’ Callen and his more jovial, yet intense, ex-Navy SEAL partner Sam Hanna, played by jovially intense rapper-actor LL Cool J. Oscar winner Linda Hunt is the “M’’-like Henrietta Lange, who oversees the team’s gadgets and budgets.

At first glance, the show’s split-level set on the Paramount Studios lot feels like it had warring decorators.

The first floor is all terra-cotta tiles, creeping bougainvillea on wrought-iron handrails, and safari-chic office furniture. But upstairs in the gleaming control room, state-of-the-art video screens dominate. Here’s where the team of attractive geeks and enforcers will piece together clues.

So the space flows from collegial to sleek in a single flight of stairs. It’s like a visual mash-up of “Melrose Place’’ and “Minority Report.’’

The posh new digs are just one indication of the network’s confidence. Since its debut as a spin-off of the military drama “JAG’’ - and thanks to surprising jolts of humor - the original “NCIS’’ has grown from just another grim crime-of-the-week puzzler into a buzzy Top 10 hit.

In an effort to distinguish the two shows from each other, and other procedurals like the “CSI’’ franchise, executive producer Shane Brennan decided to focus on surveillance technology. “I don’t think people want to sit down to watch another show where someone collects fingerprints or blood samples,’’ he says. “The surveillance technology that’s used in our everyday life that we don’t know about is pretty astonishing. You can walk across London and they can track your every step via camera. And that’s beginning to happen here.’’

The actors have fun resizing and flicking things on and off the screens of the working computers - LL Cool J is particularly adept - but know the pitfalls as well. “Your hands are tied because if it doesn’t work, it doesn’t work,’’ says O’Donnell.

Luckily, Adam Jamal Craig, who plays newbie agent Dominic Vail, is an actual IT geek when he’s not acting.

Craig won’t have to fix computers on the side if the show turns out to be the kind of hit CBS is hoping for. The network would like the show to retain the “NCIS’’ audience, if not build on it, and help launch a new 10 p.m. series, “The Good Wife.’’

The actors, for their part, talk about managing expectations. “I’ve been doing this a long time, I plan on doing it a long time and I’ve seen tremendous ups and downs,’’ says O’Donnell. “What I’ve realized is the only thing I can ever do is just work as hard as I can and put myself in a position to be successful, because so much of it is out of my control.’’

Brennan, however, is an unreserved believer. “I’m not hoping for 16.75 million viewers,’’ he says. “I’m hoping for 17.’’

The Aussie showrunner has reason to be optimistic. Most TV shows entering their seventh season are desperately introducing babies and new interns to stave off the stink of decline, not spinning off shiny new companion shows. But most long-in-the-tooth shows aren’t hitting series peaks in viewership either.

Just about everyone in the “NCIS’’ universe traces the recent surge in viewership to the marathons aired by the USA Network.

Snacking on fresh veggies at the catering table, Rocky Carroll, who plays director Leon Vance on both shows, relates a common tale.

“People tell me, ‘I broke my leg and I was home during the week and all of a sudden I started watching this marathon of ‘NCIS’ on USA and now I’m hooked on it!’ And by the time it comes to Tuesday night and the new episodes they can’t wait.’’

The same holds true for most of the new show’s cast members. “That’s how I first tuned in,’’ says O’Donnell.

LL Cool J, who had a development deal with CBS for three years, started watching when he heard rumblings about a spin-off. “Once I started watching,’’ he says, “I realized that the show was cooler than I thought.’’

Sarah Rodman can be reached at srodman@globe.com