Leslie Moonves had a one-word answer when asked if he thought “NCIS” would become a long-lived ratings hit: “No.”

“It became the little engine that could,” says Moonves, CBS’ president and CEO. “What has stunned me is how the show has grown into year seven, eight and nine, which is extremely unusual. They discovered fresh ways of telling stories. I’m really proud of them.”

“NCIS” celebrates its 200th episode tonight, a staggering achievement considering dramas often shed viewers over the years rather than draw new ones. The series is the No. 1 scripted show on television, averaging 19.8 million viewers this season and is licensed in 201 international territories.

While some TV critics brush off “NCIS” as formulaic, viewers have found plenty of reasons to flock to the series in which its writers walk a tightrope between its procedural elements — where auds can come in and out without missing a beat — and in-depth character study that pay off for its longtime and consistent fan base.

Pointing to the success of “NCIS,” TV Guide magazine critic Matt Roush singles out a cast that’s more eclectic than most, ranging from Mark Harmon’s brusque magnetism as the leading man to the sexually charged interplay between the irreverent Michael Weatherly and kick-ass Cote de Pablo. David McCallum brings the nostalgia vibe while Pauley Perrette delivers the quirk factor as the girlish Goth whose weekly peck from Gibbs is one of many grace notes “NCIS” delivers.

“Fans remember everything and they care deeply whether Tony and Ziva will ever become an item,” says Rough. “They live for the back stories of the other characters.”

Across the board, expectations were low for the military crime “JAG” spinoff series when it debuted in 2003, and that suited star and executive producer Harmon just fine.

“We were a show that wasn’t good enough to get noticed and wasn’t bad enough to get canceled,” says the plainspoken Harmon. “We shot in Santa Clarita and no one wanted to drive out there.”

Lack of initial scrutiny proved to be the best thing for the series.

“We had a lot of time to get to know each other, do this show and work on the foundation,” Harmon recalls.

And, clearly, the rock-solid element of that foundation is Harmon, the former UCLA quarterback who had a strong career in film and TV before “NCIS” took off.

“Mark is a leader both in front of the camera and behind it, and his sense of commitment and leadership translates to every aspect of the production,” says CBS entertainment president Nina Tassler. “He treats everyone with respect.”

Harmon deflects any notion that he is an indispensable part of the show.

“I know how to do the team thing. From the beginning, it was either going to work as a unit or it wasn’t going to work at all,” Harmon said.

Procedurals, in general, often have long television lives, but the nine-season run for “NCIS” is more than most.

“?’NCIS’ is the ultimate comfort-food series,” Roush says. “Critics ignore it but the loyalty and passion of the ‘NCIS’ fan is every bit as intense as that for the cult series we in the media tend to obsess on.”

‘NCIS’ 200th Episode
Dramatic growth spurt | Showrunner transition goes smoothly | Authenticity matters | Procedural also a cable, worldwide sensation