It's too bad Dick Cavett's new nightly talk show on public TV starts out with less of a shout than a mutter. In some ways, Cavett is a Great Short Hope for public television, because he could bring to it the zing and topical responsiveness it lacks.

But the first "Dick Cavett Show, at 11 o'clock tonight on Channel 26, finds Cavett coldly chummy and almost Geraldo Riverian in his deference to guests Sophia Loren and Marcello Mastrotanni. It's [WORD ILLEGIBLE] all three signed the Talk Show [WORD ILLEGIBLE] Mutual Celebrity Preservation, an unwritten code ruling out spunk and irreverence.Thus the half hour remains tame and pussylooted (the episode on which Mastroianni said a naughty word will be aired at a later rate).

Though they have both been inter-national stars for two decades, Loren and Mastroianni in recent years have found themselves up to here in trilling or preposterous film vehicles that have impaired their popularity. Mastroianni in Roman Polanski's "What?" for example, and Loren in "Sunflower" and "The Cassandra Crossing." Cavett never dares suggest that such a thing occurred, treats the pair instead like twin divinities, and dutifully plugs their new film, "A Special Day."

This approach leads to such revelations as Loren saying of the film. "I think people should and see it" and Mastroianni confessing he has skinny legs "like a bird."

At times, the two actors are charming - particularly when Loren is translating Cavett for Mastroianni, whose English remains endearingly dradful or Mastroianni for Cavett. But Cavett fails to encourage such displays of professional and personal camraderie, tending to address each guest individually as though the other weren't there.

Producer Christopher Porterfield, Cavett's long-time pal, doesn't help by placing Cavett in a chair between the two stars rather than letting them sit together and more closely interact. Perhaps this was a ploy to keep Cavett from being excluded if the result is a better, more candid look at his guests. Cavett has always lacked for selflessness; if he were a fascinating emotional dervish like Jack Paar, we wouldn't mind this. But he isn't.

Director Gordon Rigsby is even more deferential than Cavett to the guests, staying such a respectful distance from them as to avoid completely the intimate essence of good talk television. Perhaps Cavett will improve when he encounters issue-oriented guests over whom he will feel less inclined to play the titillated puppy dog.