They teach it in Interviewing 101: Find common ground with your subject. But when your subject is rich, famous, plump and happy Jack Black, that might be tricky. Do your homework and there, in the details of his 2006 wedding to cellist Tanya Haden, is the hook.

"You know, the toughest interview I've ever had was with your father-in-law."

Black takes the bait. His father-in-law? The legendary jazz bassist Charlie Haden. You need to bone up on your jazz before talking to Charlie Haden. Otherwise, he has you for lunch. Superficial "that time you worked with Pat Metheny" questions won't do.

"Man, don't just bring your Metheny game to a Charlie Haden interview," Black laughs. "Believe me, I did some research, too. Are you kidding me? I wanted to marry his daughter. I watched all 10 hours of that Ken Burns jazz documentary. But I'm still a novice. I know better than to broach the subject."

Black got the girl, and now they have two kids. And when Haden, who just turned 71, cut a CD of the traditional folk music he was raised on, a "Charlie Haden with Family" CD, guess whom he got to sing?

"Fare thee well, Ol' Joe Clark, fare thee well I saaaaay," the singing half of Tenacious D belts out. "Fare thee well, OLD Joe Clark. I'm goin' way to staaaaay.' A bluegrass standard, a classic. I've never sung that style before, but I felt I was channeling my own hickory-flavored ancestors for that."

The CD will be out at the end of the month, Black says, just in time for Black's 39th birthday.

Black sounds positively ebullient, even at the end of a long day of interviews about his new movie, "Tropic Thunder." It's a big-budget, raunchy comedy about movie stars trapped in a Vietnam War tale whose filming becomes all too real. Black couldn't be further from the character he plays, Jeff Portnoy, an obese, substance-abusing comic trapped in a series of flatulent cross-dressing comedies because that's all Hollywood wants from him.

"Look, a lot of him is me, obviously," Black says. "I haven't done multiple-character comedies that are all about farting. But I do fart in some of my movies, and I am fat, in, well, pretty much all my movies. So I understand this guy. There's a little Chris Farley-John Belushi, a little angry Tom Sizemore in there, too. I've phased the excess partying out of my life. But the guy is not a long trip for me to take."

No DeNiro "Method" stuff here, Black insists. "Yeah, I gained weight for the part. Sure."

It was a demanding movie, never more so than when Black's character slips into withdrawal and has to be strapped across a water buffalo.

"That's scary. I knew that would be when I read the script. 'OK, I'll say "Yes" to this, and then I'll talk Ben (Stiller, director and co-star) out of it.' Didn't happen. DANGEROUS. 'Get on the water buffalo. They don't mind. They won't hurt you.' Yeah, right."

Black is totally down with the New Outrageousness, the style that "Tropic Thunder" was filmed in. Raw language, substance abuse as a source of comedy, an actor in blackface (Robert Downey Jr.), this is a movie that's out there, and Black loves it.

"Let's offend EVERYBODY, and then nobody'll be offended."

"Tropic" is earning enthusiastic reviews, as is Black, who, Box Office Magazine says, is "at last reminding audiences of why they thought "School of Rock" was so funny."

No wonder Black is so good-natured, so far removed from the comically tortured funnyman he plays in the film. Happy at work, happy at home? Well, just so long as he does his jazz/folk homework. He laughs at that.

"I'm golden, man," he says. "But you? You need to study your Charlie Haden."