What we're seeing with ''You Never Know,'' the Huntington Theatre Company's season opener, is what Cole Porter intended - and it's not quite enough. Based on another play, ''By Candlelight,'' Mr. Porter intended ''You Never Know'' to be a refined chamber piece. Its producers felt differently and clogged it up with extra characters and songs by other composers. Its Broadway run was brief. The script - which was lost for years and finally turned up among Porter's papers - was dusted off by Paul Lazarus (its current director) for his 1982 production at the Dorset Theatre Festival in Vermont. Except for some adaption by Mr. Lazarus, this is the real play.
That said, I wish there had been more to the book to justify the effort. It's a lightweight bit of fluff about a songwriter cad and his affable butler who exchange clothes so that the butler can woo a lady (who also isn't who she seems) in style. It's the kind of farce in which everyone freezes every time the doorbell rings. Charming, but thin.
The performances are fine - the 1920s panache is laid on with a trowel, the butler (Mitchell Greenberg) has the light, toothpick grace of Fred Astaire (especially when he's dancing with the telephone), and one of the girlfriends (Carolyn Casanave), a Bronx Betty Boop, says ''oar-vwear,'' instead of goodbye.
John Falabella's costumes shimmer, and the sets, by James Leonard Joy, are a deco delight with a pearlized luster that changes color with the light. But the beloved Porter hits, ''Let's Misbehave'' and ''At Long Last Love,'' end up being diamonds among this rhinestone of a plot.