Mark Harmon, Michael Weatherly Mark Harmon, Michael Weatherly

Tony DiNozzo is being utterly condescending to Leroy Jethro Gibbs for the very first time — and the last. Shooting an NCIS interrogation scene unlike any other, Michael Weatherly and Mark Harmon are in flashback mode, circa 2001, facing off as captor and the captured. An abandoned newspaper office building in downtown Los Angeles has been transformed into a Baltimore police station, where DiNozzo has brought a handcuffed Gibbs in for questioning after tackling and arresting him on the street. When he learns that his future boss is not a crook but an undercover NCIS agent, he is dumbstruck — by Gibbs' seeming stupidity in getting busted.

"Well, it's official," DiNozzo says, as his then partner takes the cuffs off Gibbs. "He's a cop — a Navy cop." Weatherly starts to improvise as the takes progress. "Navy NCIS," the actor adds, in as patronizing a tone as possible — an inside joke for fans, since that was the silly full title the series briefly bore during its first season in 2003-4. "So what do I call you? Leroy? Jethro?" Adding a line that may not make the final cut, Weatherly snips, "What is this, The Beverly Hillbillies?"

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Harmon does some riffing on the script, too, but true to the actor's close-to-the-vest minimalism, all his improv is visual. His famously caffeine-addicted character walks to a coffeemaker, pours himself a cup, registers a moment of disgust and tosses it in his disapproving interrogator's wastebasket. It's the beginning of a beautiful friendship, all right.

NCIS has been and always will be an ensemble show, not a buddy series, but if there's one core relationship that's been at the heart of the comic drama through eight increasingly successful seasons, it's the peculiar bond between Gibbs and DiNozzo, which intrigues on several archetypal levels. "In addition to the student/mentor relationship that exists," says exec producer Gary Glasberg, "there's a father/son aspect to it, a big brother aspect to it and a straight man/comedian aspect to it."

A deeper insight into all those may be provided by the flashback episode, "Baltimore," airing May 3. It's like a superhero-origin story, except instead of radioactive spider bites, we see how Tony picked up his suave clothing tastes, along with what impressed Gibbs enough to hire an impulsive detective who would quickly become his No. 1 son.

And — naturally — we witness the mother of all proto-head slaps across Tony's noggin. Which is not so bad a form of comeuppance, given the initial body slam.

"So much of what we do here is based on trust," says Harmon, taking his lunch break with the crew. "I don't know if you're ever going to be sitting down with some other cast at Year Almost 9, heading toward 190 shows and talking to them about the mutual admiration society they share in their workspace."

Their trust-based improv goes back at least as far as the very first head slap — in real life, not this new fictional backstory — which occurred during the filming of the fifth episode in '03. "It was totally spontaneous" on his part, Harmon says, "though I've heard a lot of people claim it! Michael was doing his thing to a young Navy female petty officer on a ship, and it just seemed appropriate at the time to bring him back toward some sort of reality, which certainly the head slap did." The fans loved it, and it became a recurring sight gag. According to one site's estimation, there were at least 23 head slaps in Season 3 — some between other characters — but it's been brought down to a more reasonable average of three for the past few years.

The regulation of head slaps points to an overall balance of tone that has proven tricky — but successful — for NCIS. Weatherly is surely the only actor in any medium who is constantly compared to both James Bond and Jerry Lewis. He looks swanky in a suit — and as Weatherly notes, "Baltimore" is the first time in 40 episodes he's gotten to wear casual dress on the show. He's credible as a man of action and when he's brooding, even after you've seen him dress up in a fat-Elvis getup. But some fans wondered if the character's competence was being sacrificed for comedy.

Weatherly did, too. "I have been more alert about: Are we going to step on Tony and put him in a position where he suddenly doesn't know how to fight very well?" When Tony got the sillies, "I've always thought, well, it'll be fine — until this year. In 2011, I'm like, no! We've got to protect him a little. But I think the writers were having the same idea."

For more with Harmon and Weatherly, pick up this week's issue of TV Guide Magazine, on newsstands Thursday, April 28!

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