IT'S AN accepted piece of common American knowledge that Singin' in the Rain is one of those classic 50's musicals--perhaps the classic--what with its Adolph Green-Betty Comden score and its Gene Kelly-Stanley Donen choreography. A lot of people who have never seen the movie in its entirety have managed to catch a tantalizing glimpse of the title number--three or four minutes of sheer, unadulterated joy, with Gene Kelly splashing around like an ecstatic, acrobatic duck through foot-deep puddles and grinning idiotically under a rainpipe's torrent.

Now, after 23 years, MGM has decided to rerelease the movie nation-wide--with plenty of hype, including umbrella-festooned theater lobbies--and give those people a chance to see the other 97 minutes. MGM would have done better to confine itself to three minutes of Gene Kelly in the puddles. Unveiled in all its technicolor gaudiness, Singin' in the Rain stands revealed as an overblown, badly acted, often tedious extravaganza that almost drowns Kelly's perfect little droplet.

Kelly himself is at least passable as a 1920s silent screen idol making the transition to talkies, and of course he's much more than passable when he's dancing. But the rest of the cast is just too much. The same heavy-handed twenties hamming that the movie ostensibly spoofs seems to have infected their acting style. Debbie Reynolds starts out insulting Kelly with all the petulance of an angry kewpie doll (and of course for him it's love at first sight, since she's "the first dame who hasn't fallen for his line since he was four"), but she ends up melting in his arms with Shirley Temple sweetness. Donald O'Connor is so frenetic as Kelly's comic sidekick that he's exhausting to watch, particularly in "Make 'Em Laugh," a tribute to vaudeville slapstick during which he walks into walls, falls over couches, and generally mutilates himself in a (vain) attempt to make someone, anyone laugh. But Jean Hagen is the most annoying of all, doing a pale imitation of Judy Holliday as a shrill, dumb blonde, a silent star who refuses to admit she wasn't cut out for the sound era.

None of these characters has any more than two dimensions--Hagen's dumb blonde should have at least a spark of pathos, but instead you cringe every time she appears on the screen. They're like card-board cutouts, decked in lavish twenties satins and furs, pasted against larger-than-life sets, mouthing jokes that aren't funny, singing and dancing on the all-too-obvious cues.

Even the singing and dancing is kind of a letdown--it's good but not fantastic, certainly not "classic"--with the exception of an extended balletic sequence called "Broadway Melody," in which Kelly and Cyd Charisse slink around with lithe, animal sensuality. The ballet is a self-contained entity that has nothing to do with the plot, and its unmistakable 50s jazziness is completely out of the movie's 20s context. Kelly seems to have had a good idea for a dance number and just thrown it in as lagniappe, but it stands up well on its own.

Lagniappe is about all you can expect from this movie, and while we're on the subject, the movie theater it's playing in is almost worth the price of admission. It looks like a church, which isn't surprising because that's what it was built as. Then in 1913 or so it was converted into a legitimate theater and has since had a close brush with demolition in order to make room for a parking lot. Now it's being carefully brushed off and restored, its wood panelling, brass railings, velvet curtains and stained glass, window preserved for posterity. It's also probably the only movie theater that plays Gilbert and Sullivan between showings. But if you want to preserve any illusions you might have about Singin' in the Rain, maybe you should wait until the feature has changed before you check out the Exeter Street Theater.