NEW YORK As you watch Chris Lilley playing all three leads in his new mockumentary series about an Australian high school, you can expect to have three responses.

First, you're amazed that, even as a stunt, he would dare to try this repertoire of roles.

Then, quickly enough, you notice how comfortably he channels each personality: an unruly misfit named Jonah; a snooty 16-year-old, Ja'mie; and Mr. G, the showoff drama teacher.

Then you simply forget that one gifted actor has brought these characters to life, and done it by pure force of performing (Lilley abstains from fancy costume and makeup effects, even in depicting Ja'mie, which is a pretty good trick since she's not only a girl, but also a self-declared hottie).

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The mythical school where this trio of tales is set lends its name to "Summer Heights High," an eight-part series created and written by Lilley. A hit last year in his native Australia, it premieres Sunday at 10:30 p.m. on HBO.

The series is richly funny as it makes the most of its "documentary" framework: Subjects discuss themselves for the camera, then are soon contradicted when the camera catches them in "candid" behavior. Self-delusion is epidemic at this school, where Mr. G reckons he's an inspirational figure, however narcissistic his song-and-dance demonstrations, and where even Ja'mie's tossed-off pleasantries betray her meanness ("Oh, you've got wheelchair people! That's so cute!").

To play Ja'mie, who arrives as an exchange student from her posh private girls school, "I knew I wanted there to be a contrast between this girl who everyone thinks is wonderful and amazing but who's really nasty and selfish," Lilley says.

Lilley created Mr. G during a stint as a standup comic. Now the character is in full flower as a small-time spectacle of oversized ego who composes a song for the student musical celebrating his own presumed virtuosity: "My name is Mr. G/ Three talents one, two, three!/ Singing and dancing/ And acting is my life!"

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To flesh out the characters, Lilley spent months embedded in numerous schools, gathering material and soaking up the atmosphere.

"I wanted to make sure I was very current. I didn't want the show to be an old person's perspective," he says. (Lilley, who turns 34 on Monday, has a cherubic face and bright blue eyes that make him look at least a decade younger.)

"You have ideas that you just want to confirm," he goes on. "You write a scene about a staff meeting, but you want to sit in on a staff meeting, and make sure all the details are right."

Assisted by Lilley's eye for detail, "Summer Heights High" feels oddly authentic and convincing, and it packs more than laughs. It has a poignant, even painful side.

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This is thanks mostly to Jonah. Here, at his third school in 18 months, he emerges as the series' truly sympathetic figure -- notwithstanding his propensity for f-bombs, rowdiness and defacing property.

Jonah's lewd graffiti "tag" became a conspicuous part of the redecorating scheme at the shooting location, Brighton Secondary College in suburban Melbourne.

"It's a quite nice-looking place," says Lilley, "but I was very keen to make it appear that it was a lower-income school. So we made it look quite ugly."

Classes were in session during the three-month shoot, with Brighton students in nonspeaking parts.

At first, Ja'mie raised eyebrows in the hallways.

"We got a few funny looks: 'What is the tranny doing in our school?'"

But the character's full acceptance by the girls who play Ja'mie's in-crowd friends is palpable, giving further credence to how Lilley portrays her.

"I told them, 'You need to believe I'm real. Treat me like a real girl.' And they did." It was the same sort of chemistry among Jonah and his mates. "Those boys knew who I was, but they treated me like I was Jonah. It's a strange crossover."

Lilley, who had earlier success playing five characters on his 2005 series, "We Can Be Heroes: Finding the Australian of the Year," claims to have no interest in roles that don't inhabit a world he masterminded.

"I get asked all the time to audition for things, and I never do it," he says. "I'm much more interested in creating my own projects. I think I'm better at conceiving the whole project and seeing it through."

By all signs, Lilley is an original -- or means to be. When asked to name an actor or comedian who influenced him, he seems thrown by the question.

"I don't want to be like anyone," he replies. "I want to just do my own thing. I want to be like people who do their own thing."