‘APART FROM the known and the unknown, what else is there?” asks the playwright Harold Pinter.

THIS IS what I know. When you ask to go backstage to congratulate any or all of the cast members in the musical “Xanadu,” you are advised that they dress and undress together in a very small space and there’s no room for anyone to “visit” with them.

So last week, when some of us wanted to greet Whoopi Goldberg after the show, we and our menfolk had to stand near the stage, still in the audience, after the crowd filed out. In time, the cast trooped out to mix and mingle with us. There was the sexy all-American leading man, Cheyenne Jackson, who declared, “Well, tonight was sure boys’ night out in this theater. Oh, hell,” continued Cheyenne, who’d had his mighty legs in short shorts on display all evening, now covered up, “every night here seems to be boys’ night out!”

The mighty Mary Testa, who blows everyone else off the stage as the “evil” Melpomene, appeared in our midst as sweet as pie. She makes a formidable “pal” onstage for our Whoopi, who doesn’t go about on roller skates, as I’d thought, but underplays dramatically with Testa as another evil genius, Calliope. (Their ridiculous interplay with some of the audience, who actually sit onstage throughout, is just the funniest.) I adored the show’s leading lady, Kerry Butler. This little blond minx of a muse called Clio, tiny as she is, remains the staunch thread that holds this entire “plot” together.

The story is about Greek gods coming down to mix with mortals in Venice, California. So, shades of the genius Cole Porter, who often used the same idea. While “Xanadu’s” plot and music don’t come up to the classic Cole, there is a distinct similarity here to Porter’s “Out of This World.”

Cheyenne plays the manly, ’80s-era hippie mortal who doesn’t “get” pop-culture references about such bygone stars as Errol Flynn, or mentions of “vaudeville,” and other arcane matters.

I was hoping to see Tony Roberts, who segues between playing Zeus and a hard-driving old businessman, but he must have escaped through another exit. He is merely marvelous dancing, singing and bopping around, playing in emphatic contrast to his youthful, whizzing, “gee whiz”-type roller-skating cast members.

By now we had added the grand Oscar-winning actor Joel Grey to our party. He’d been in the audience to see Whoopi. When Ms. Goldberg appeared, she said she was soaking wet from her exertions and was raring to get out and get home. But she stopped to kiss us and show us some new shoes she had discovered. (Whoopi always has some fashion accessory or artifact she has just uncovered!) We all burst into applause for this giant, big-hearted star before she went on her way to raging crowds awaiting her at the stage door on 44th Street.

What a night! “Xanadu” is SRO only these nights, but even after Whoopi departs to go back to normal, this will be the musical on Broadway you want to go see if you simply want to have the time of your life.

THE RED-HEADED Arlene Dahl is just about “The Last of the Mohi cans” from MGM’s glory days and celebrated her 80th at Swifty’s with 40 famous friends and plenty of others who were “cryin’ to get in.”

This lady has had five marriages and is replete with handsome children and a clutch of grandkids. She has found happiness in a 25-year wedlock to NYC’s designer Marc Rosen and the evening’s spoken tributes all included him. Many raved about Arlene, who starred with Fred Astaire in “Three Little Words,” to name one triumph. Rex Reed spoke, saying Arlene had been his favorite since he was 8 years old, growing up in Texas.

I especially liked Michael Feinstein coming by to sing a naughty Irving Berlin version of “You’re the Top.” (Example: “You’re the tits of Venus!”) Joan Rivers and Elaine Stritch sent best wishes from London.

HOLLYWOOD said so long last week to the formidable Bernie Brillstein, agent, manager, coddler and generator of talent. He was the last of his kind, maybe the first! I’ll never forget meeting this genial guy, dancing the night away with him in Studio 54. After, we remained tight. I never had a problem he didn’t try to solve for me. Surely will miss this man who helped make the Muppets, John Belushi, Gilda Radner possible.

YOU SAW the weightlifter at the Olympics who dislocated his elbow? That’s how I felt trying to lift Vogue for September. (So what is all this about there being less advertising in mags?)

Don’t miss Billy Norwich‘s take on manners during an election campaign, if you can find it on Page 482. Billy claims, “Don’t ask – ‘Who are you voting for?’ ” Billy says, “It’s invasive. Better to say, ‘How do you feel the election is going?’ ” Hmmm, I think either way you may still get slugged.