On a soundstage tucked within a Santa Clarita industrial complex, cast members of CBS' "NCIS" are rehearsing a fairly simple scene.

Mark Harmon as Special Agent Jethro Leroy Gibbs, Sasha Alexander as former Secret Service Agent Kate Todd, Michael Weatherly as agent/classroom troublemaker Tony DiNozzo and Sean Murray as newbie computer geek Timothy McGee theorize on the disappearance and fate of a missing naval petty officer.

All is going smoothly when Alexander, almost to herself, says, "We're gonna finish this in no time."

As one, her colleagues and surrounding crew members react with mock outrage.

Alexander has breached one of the production's cardinal rules - never speculate on a scene's shooting length, lest you jinx it. Something called the "Dobbie board" - merely a stick with its moniker scribbled upon it, named after the crew member who first committed the offense, attached to a string - is produced.

Weatherly hoists the petite actress upon her character's desk as the string circles her neck and a Polaroid camera appears to immortalize her transgression as laughter echoes through the cavernous edifice. A wall on "NCIS' " stage memorializes those who preceded Alexander in this breach of etiquette.

Work on the "NCIS" set belies the tension-filled world of high-stakes network television. "NCIS" (the letters stand for Naval Criminal Investigative Service) is a quiet hit going mainstream. In its second season, it has become the most popular program on Tuesday nights ("American Idol," returning tonight, likely will change that).

Its audience has increased by 2 million this year, and a whopping 33 percent in network TV's all-important 18-to-49 demographic. All earned the old-fashioned way, through solid storytelling and, most important, colorful characters.

Despite good reviews, "NCIS" was initially dismissed as both a spinoff of military drama "JAG" (also created by Don Bellisario) and a copy of "CSI," but has emerged as something significantly more - a genuinely funny crime drama in which viewers are lured more by character interactions than the mysteries' plot twists. The production's behind-the-scenes playful camaraderie is vividly apparent in what shows up on the air.

"This show isn't cooked up by a committee of people trying to decide what will meet with the approval of the audience," says Weatherly. "The title (with its "CSI" echo) might have been cooked up - (but) by the Department of Defense.

"It's really an office drama," he continues. "We bitch and moan the same way anyone who works at a pharmaceutical company does around the water cooler. We're not superheroes or fashion icons, it just happens that what we do is interesting. But mostly we're just bitching and moaning about things."

"This goes all the way back in style to 'Magnum, P.I.'" says Bellisario, referring to the first hit series he created a quarter-century ago. "I tell writers our show is about what happens in the cracks of the regular shows. 'NCIS' is not about following forensic procedure, it's about the characters quipping back and getting into little arguments."

Bellisario - whose visage is among those on the squad-room set's "10 Most Wanted" wall - says he pleaded with the network not to market the series as a "JAG" spinoff. "Which they did," he laments. "I'm probably the only creator of a series who said, 'Please don't put "By the creator of" on a billboard.'

Harmon notes that Bellisario is a stickler for character nuance, recalling with a chuckle that when a crew member placed an authentic NCIS coffee mug on caffeine addict Gibbs' desk, "That lasted one day."

"I went through the roof," Bellisario agrees with a laugh. "It's in the details to me. Gibbs drinks out of a Starbucks or Coffee Bean cup. Strong, black."

And the cast responds with equal dedication to capturing their characters. David McCallum, who plays medical examiner Donald "Ducky" Mallard, has done so much research into coroners that, Bellisario jokes, "He's probably slipped into a morgue and dissected some bodies."

"You do have to be careful" about obsessive research, McCallum concedes. "You can be at a dinner party, talking about how you section a brain - you don't want to put people off. But I find they're interested - people will say, 'Gross!' - and then keep asking questions."

Bellisario says the cast makes it look easier than it is. "This show is very difficult to write to my satisfaction," he says with a sigh, admitting that shooting begins on episodes before a final script is in place. "There are only one or two writers who get it. Some can do the forensics, some the story, some the humor, but it's hard to get it all to mesh."

Harmon is impressed, nonetheless, with how the series has come into its own. "You can say it's luck, or you can say someone's pretty smart - no question Don's pretty smart," he says, adding, "There's nothing saying that you can take (actors) from different places and put them together and they're gonna get along. I've been on sets where it's hell. It's rare to be on a set where they get it. Here, they get it."