The word going the rounds (no doubt started by NBC) is that Cavett's too intellectual for the video masses -- and about his brace of fans. . .strictly New York Review types. Granted, he's not your usual every night M.C. (nary a Gabor on the show), still, his metier is talk -- good talk, literate (except with Mailer and Vidal), fun (ditto), sometimes stimulating (as with I.F. Stone), occasionally awkward (when he slips and calls Jan Morris ""sir""), never banal and almost always entertaining. Exactly how you'll find him in print -- which makes one wonder why the so-so gimmick of a turn-around interview. Born in Nebraska and educated at Yale (where he seems to have been almost as serious about his studies as about bird dogging celebrities -- a fascination never lost), Cavett bears the impress of both, along with a self-protective toughness developed via long periods as an unemployed actor-comedian, pounding an office temps typewriter, also pavements -- until finally, being pushier than you'd guess, brazening a job on the Parr show. And the rest, as they say in the trades. . . . Never big on the Nielsen, this time out Cavett's bound to place on that other more prestigious chart.