Documentaries, even fake ones, are all about inquiry, and watching Guest's characters (especially Corky St. Clair, the neurotically deluded and inept theater bitch maestro of ''Guffman''), you don't just giggle -- you fall in love with their self absorption, their heroic second rateness, their hidden shades of madcap myopia.
In Best in Show, Guest's new comedy about the Mayflower dog show in Philadelphia and the infantile middle class eccentrics whose pets compete in it, you giggle every so often, but you never give yourself over to the characters, and that's because Guest hasn't discovered enough layers in them. He's reassembled most of the brilliant improvisational players from ''Waiting for Guffman,'' but this time they're cast as two bit fruitcakes whose humorous soft center is the projection of their vanity onto their pooches. Since that rather unoriginal satiric flaw becomes obvious, in most cases, within 30 seconds, the film has little to do but repeat the same giant bull's eye gags over and over again.
The picture hits an overstated note from its opening scene, in which Parker Posey and Michael Hitchcock appear in a therapist's office as a hysterical couple who dote on their weimaraner as if it were a real child. Shrink jokes are old hat, as are ''dog owners as mommy and daddy'' jokes, and can we really buy that both of these two would be wearing braces?
Guest has better luck with an acerbic gay pair (Michael McKean and John Michael Higgins) who are more primped and primed than their shih tzu, but Eugene Levy, as a terrier owner with two left feet (literally), just does a tamer riff on his ''Guffman'' putz. Guest himself is content to melt into the ensemble this time as Harlan Pepper, a North Carolina fishing store owner who talks to his prize bloodhound in a courtly James Carville drawl. Harlan is silly in an oddly dignified way, and that's about all there is to him. Guest gets brownie points for modesty, but ''Best in Show'' feels stranded without a protagonist, a mocking center of gravity.
At the dog show, Fred Willard shows up as one half of the broadcast team, and the film suddenly bristles to vulgar, antic life. Willard's Buck Laughlin is one of those spectacular American ignoramuses who will say anything and assume that it's a showstopper simply because he thought of it. He knows nothing about dogs, but that doesn't keep him from theorizing out loud (''It's terrible to think that in some countries, these dogs are eaten!''), as his British partner looks on in barely muted horror. Willard has the slingshot zest, the unruly force of ego, that's missing from the rest of ''Best in Show.'' This one, I'm afraid, just goes to 5.