The music business uses the phrase mastertape. In the film business the word for “Jersey Boys” is masterhit.

Kathrine Narducci, playing a mother: “Clint Eastwood keeps a Zen set. It’s like family. He’s so unHollywood you just want to hug and kiss him.”

I asked why direct a jukebox musical. Said Clint: “Music comes back with a vengeance. It can return and hold up for years. I was raised with this music. Look, life doesn’t always go the way you plan. ‘Jersey Boys’ was a mystery to me. I’d heard about it for a decade. It tried out in La Jolla. Nobody thought it would go anywhere. On Broadway it stayed nine years. So I went to see it.”

Calm as a swami amid a premiere party clatter — people clucking, glasses clinking, cameras clicking, so is Eastwood really Zen? “Life is not worth getting upset.”

Chris Walken: “Mine is the mentor role. I never saw the Broadway show. But I knew their song, ‘You’re Too Good To Be True.’ In our ‘Deer Hunter’s’ poolroom scene it played over and over.”

John Lloyd Young: “I did 1,400 Broadway performances as Frankie Valli. The movie’s different. No projecting to the last row. A camera’s in your face. And the film goes deeper. Onstage Valli’s an emceelike teacher. With Clint, it’s more observing the audience. We sang live. One day all day we redid ‘Cherie.’ In the Ahmanson Theatre lobby with 300 extras like a supper club, we kept doing ‘Can’t Take My Eyes Off You.’ ”

Screenwriter Marshall Brickman: “When they mentioned ‘The Four Seasons,’ I said, ‘Hotel or restaurant?’ I was a banjo player after college, but Beatles and Beach Boys stories went no place so I thought this couldn’t work. Here all four had been mobbed up. Interviewing was like ‘Rashomon.’ Each memory, different. Each said of the other guy’s story: ‘Naaah, that’s s - - t. Here’s what happened.’ ”

Guests Alan Cumming, Tovah Feldshuh, Melissa Leo, Clive Davis wandered by. Also Roger Rees: “Remember me from 8 ½ hours long ‘Nicholas Nickleby’ which played on Broadway?” He in this, too? “No, I’m eye candy. My partner’s co-writer Rick Elice.” Also swanning around, Richard Belzer whose plus-one in the theater was his dog.

There was Steve Schirripa who plays Vito, Clint’s daughter Francesca playing a waitress, someone named Joey
Russell (or maybe Russo) in the role of Joe Pesci. The place was full of guys, young, similar-looking, not superfamous. I couldn’t tell one from another so if I get them mixed up . . . so? So do me something. By next year, by the time they all win Oscars I’ll get it straight.

Mike (I think) who plays the manager (I think): “I knew everything. Every song. Every move. I saw the show three times. Clint said: ‘Think of this scene as your big gun shoot. Look down at your rifle, target’s in sight and go out with big blazing shots.’ ” OK?

“Boardwalk Empire’s” Vincent Piazza: “On set Valli gave me anecdotes and relationship stories. I wasn’t in the play. After I got cast I saw it from the fifth row. I couldn’t sing so a voice coach took 30 days to teach me. Why they cast me when I couldn’t sing, who knows. Maybe they had a shortage of Italians.”

The party was in the Plaza’s Angelo Galasso shop. He explained Beckham, Diddy, Pacino, Randy Jackson wear his couture. “In the film Erich Bergen wears my clothes.” P.R. lady Peggy Siegal was also inside his striped jacket. This he didn’t mention.

The movie? Marvelous. Even Richard Belzer’s dog gave it four barks.

PAY attention. The Plaza Hotel’s public Ladies Room sign explains it’s open “Until 11 p.m.” So this is to tell anyone who has to pee around midnight, lotsa luck. Try the Waldorf.

Only in New York, kids, only in New York.