STILL TRYING to cash in on the audience Johnny Carson attracts. ABC has put a new witty and charming host in the time slot opposite Carson. At first glance, the new Dick Cavett show seems to be just another Tonight Show imitation-like the Jocy Bishop show it replaced. But Dick Cavett's approach is somewhat different from Carson's. Unlike the often-famous Carson, Cavett encourages serious conversation on his show-Vietnam has been a key topic on several nights-and this alone may indicate that his show is not going to be another slick late-night confection.

The Tonight Show's attraction rests on its atmosphere of polish and color, an atmosphere created in no small part by the shimmering set, the flashy clothes which Johnny and the bandleader wear (Johnny's wardrobe by Ol?g Cassim), the stylish bandleader. What people say on the show is fairly unimportant: deliberately, Carson keeps the conversation light, and-like the show itself-trivial.

Very much unlike Carson, Cavett allows his guests to speak for quite a long time-some as long as twenty minutes-so that they can express their views in depth. The content of each person's comments is highlighted by the fact that there are never more than four guests on each night, in contrast to the five or six who crowd the Carson stage on any given night.

Although Cavett insists that his guests represent all political viewpoints, it is clear that most of the people on his show take a critical stance toward America. Last week, for instance, his guests included: Townsend H?opes, whose book. The Limits of Intervention, discusses the mistakes the government made which got us into Vietnam: Harvard's own George Wald and columnist Jimmy Breslin, both of whom have been outspoken (in different ways) about America's faults: and Jonathan Miller, one of whose topics was myths Americans have created about themselves. The only guest who came close to balancing out the list was Kevin Phillips, anther of The Emerging Republican Majority and an important assistant in Nixon's Justice Department.

THE MOST serious fault of Cavett's show, unfortunately, is Cavett himself. He is charming-like Carson, boyishly charming-but he seems disappointingly simple-minded on his show. He may have gone to Yale, but he listens smilingly to what his guests say with an apparent naivete reminiscent of Carson's apparent naivete. He acts as if he is positively unaware of the meaning of what anyone is saying.

It's not likely that Cavett is really that dumb. Perhaps he acts that way because talk-show hosts are supposed to be innocuous, or because he is trying to disguise the fact that his guests are talking about serious subjects. Or he may feel that it would be inappropriate to appear to agree or disagree with a guest depending on what kind of stance that guest is talking.

And so, for whatever reason, he deliberately appears unresponsive while he asks Townsend Hoopes questions about our foreign policy which he knows will encourage Hoopes to talk critically about the government. And he smiles continually while talking to Kevin Rhillips, whom he might despise.

The approach can be irritating when-after Phillips comments that his book has nothing to do with new Republican programs but is only concerned with facts-Cavett does not point out that, being in the Nixon administration, Phillips might be interested in making use of these facts to create an emerging Republican majority.

Some day Cavett will stop feigning ignorance, and there already may be signs that he is beginning to do so. Recently, Cavett showed that he will not take interruptions by commercials quite as blandly as everyone else does. Last Thursday, when the man at the piano began to signal that it was time for a commercial, Cavett demanded that he stop playing and allow Phillips to answer a question.

THE REALLY important question, however, may be whether Cavett's show can stay on the air. Will the members of the Silent Majority put up with the kind of political criticism Cavett is in fact encouraging? Moreover, do they want to watch a midnight show which contains serious discussion of any sort?

Watching Cavett's show is not always easy. It takes a certain amount of concentration to watch anyone speak for twenty minutes-especially on television, which is not geared for unbroken action or conversation (as Carson must well know). Most people who twirl the midnight dial are probably looking for an easier-to-watch show like Carson's before dozing off.

Cavett has already shown that he has trouble appealing to the mass-consumption audience. Twice before-once with a daytime show and again last summer with a variety hour-Cavett put on the same kind of solid performance, only to meet the network axe when he couldn't bring in the soap commercials. Running against Carson's show, the fiscal prospects look bad again. Cavett's fans can only hope that ABC looks past the ledger books when it decides whether to keep the show.