Mark Harmon's looks nearly lost him the lead role on "NCIS," the forensic drama with a military flair now in its third season. Series creator Donald P. Bellisario had written the main character, Leroy Jethro Gibbs, as a tough-minded former Marine gunnery sergeant but had no specific actor in mind for the part.

"Someone mentioned Mark Harmon and I said, 'He's a pretty boy; I just don't see him as this character,' " Bellisario said. But the producer reversed his thinking after watching Harmon in a four-episode 2002 story arc on "The West Wing."

"I had envisioned the boyish Harmon, who was too good-looking, but I saw he now has a maturity to him," he said. Harmon was cast as Gibbs, whose investigative instincts and unblinking determination are lightly salted with extra-dry humor. His no-nonsense manner drives the team at the Naval Criminal Investigative Service in Washington to solve a barrage of cases — including forensic mysteries, bomb threats and cases involving missing children.

"As an actor, you don't always know where a role is going, and part of the fun of playing this guy is that it changes and he continues to challenge me," Harmon said of his character, whose occasional hints of past secrets intrigue his staff — and viewers.

Bellisario, who also created "Quantum Leap," "Magnum, P.I." and "Airwolf," said: "Networks and studios always want to know a character's heritage and details, but I like to let those things grow with the show."

Since its 2003 debut, "NCIS" has been a steady performer, ranking consistently among the top 20 prime-time shows.

"NCIS" stemmed from a two-part episode of another Bellisario show, the now-defunct "JAG," in which Gibbs and three other "NCIS" characters were introduced when they investigated a murder.

"I recognized this as a strong character piece, where there is always a caper," said Harmon, who also appreciates the small complexities of his role.

"In one episode, Gibbs goes to tell a woman about her husband's death, and you see this young boy in a half-finished treehouse. So that's symbolic — the dad's not coming home to finish it ...," Harmon said. "You see how Gibbs deals with kids better than adults because he is a kid in some ways."

Doubts about casting Harmon are long past. "Mark is so into the role, he whitewalled his hair," Bellisario said, referring to a hairstyle in which the sides and back are shaved almost bare. "He said, 'You think I like wearing my hair like this?' But he said Gibbs would wear it that way, and that's enough for Mark."

Harmon, who averages long days along with the rest of the cast during production, said: "I've done other shows where you're out there trying to get people to watch and you're apologizing for the show. ... It's enjoyable that this one is working so well."