Young, Gifted and Jack

From frowning to flatulence, Jack Black uses every faculty to generate his particular brand of comedic action. And nobody is safe in his presence.

We're in a New York guitar studio where the comic actor is strumming a Gibson acoustic with co-star and best friend Kyle Gass. Suddenly, they explode into a "Nightline" rock anthem that starts with "Want the scoop/look at the poop/Try to figure out some things ..." and finishes with "It's Martin/kicking up the news jams."

It's a funny, harmless lyric and consistent with Jack Black's zany yet empathetic persona -- an actor who might be suffering eyebrow twitches and attention deficit hyper disorder, but is warm hearted and harms nobody in the process.

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Black and Gass are currently on tour promoting their new movie, "Tenacious D: The Pick of Destiny," a film about their cult rock band that they formed 12 years ago after meeting as budding performers at the Actors' Gang in Los Angeles.

Black, who looks much younger than 37, says he's more proud of this film "than any other film I've been in." And he's been in some pretty big movies.

Black's big break came in 2000 when he won a part in "High Fidelity" as the acerbic record store manager who refused to sell what he deemed to be mediocre music to his customers.

But he almost didn't take the part. As Black explains, "That was the biggest part I'd got in a film, and I was very nervous. I turned it down initially because I thought ... 'what if people don't like me? I just rather keep the little piece of pie that I have over here.'"

But he was persuaded to ignore his insecurities, principally because director Stephen Frears kept offering more money. The result was a movie that catapulted Black out of obscurity. Many critics thought this performance stole the show from established stars, like John Cusack who also appeared in the film.

"School of Rock" soon followed and was a smash. Even the DVD sold out. It allowed Black to join that rare breed of comic actors who can carry an entire film on their own shoulders.

"Nacho Libre," released earlier this year, was more of the same with Black playing the role of a chef in a monastery-come-orphanage for children in Mexico. He decides that the only way to improve their diet is for him to become a wrestler and earn cash to buy better produce.

But again, Black suffered anxiety about whether he could pull it off.

"I was freaked a little bit with 'Nacho Libre' because I was playing someone from another country. I've never done that before and I was like, 'what if the people from that country are offended and they're thinking, hey, is he making fun of us?' So, I paid extra attention to getting the accent right and, you know, doing some research and ... I felt good in the end that I did it."

Black's insecurities have manifested themselves in other ways. He says that he has a combined form of obsessive compulsive disorder and ADHD. "I have a little bit of OCD and ADHD. So I'm OACDH!" He also admits that these symptoms help inform his performances.

But now, having become this silly yet sensitive actor, he's decided to indulge his passion for rock music and his own band Tenacioius D. And here's where the trouble may start: having built a reputation as a family entertainer, the content and dialogue of the new film is overtly, almost obsessively sexual.

"As Freud would say," explains Black with tongue firmly in front of his wisdom teeth, "it's all about sex. It's all about the penis. It's all about the vagina."

"Tenacious D: The Pick of Destiny" is somewhere between an adolescent fantasy and a drama-documentary. It follows a youthful Black who infuriates his fundamentalist father, played by Meatloaf, because he wants to be a rock star.

Black runs away from his Christian parents to Hollywood and meets an apparently accomplished musician -- Gass.

The duo join forces to go in search of a legendary guitar pick. If they find the pick, then their dreams of becoming the best band in the world will come true. The pick itself was originally formed from the tooth of Satan, but is now displayed in a rock music museum.

Black decided to forgo his usual fee of around $12 million, accepting less than a tenth in order to get the film made. And he says it's allegorical of much of his own life.

The film ends in a heavyweight rock-off between the band and Satan. Black says that he wouldn't be the actor he is without his own set of demons.

"I guess maybe I like the danger. I like to act the extremes. And I like crazies. I've always been fascinated by performers or just people in general that seem a tiny bit insane. That's a key ingredient for my enjoyment."

Insanity, I ask him?

"Yeah," says Black.