LUNA -- At the Avalon.

It is no insult to Jill Clayburgh to describe her acting as soap operatic. The prospect of steady employment lures good actors to daytime television, where an acting style you could call Passionate Realism has been perfected.

Clayburgh, a "Search for Tomorrow" alumna, is a master of this style. It consists of lacing high emoting with small, everyday gestures, so that moments of exaggerated crisis are lent verisimilitude by being associated with the ordinary. One Clayburgh standard, and it's a good one, is produce a quavery smile just when she is about to explode. If she was playing Medea, she would give Jason a sad, brave, apologetic little look before letting him have it. This approach works wonderfully for moments of modest modern tragedy, such as being wronged, in "An Unmarried Woman" and "Starting Over."

It's not enough, however, to cover the monstrous deficiencies of "Luna." To say that Bernardo Bertolucci has made a film like a soap opera is intended as an insult. It's one just one unmotivated clash after another, a catalogue of big-time psychological messes, put together with nothing coherent in between.

No amount of realistic acting could put over the medically implausible idea that the way to calm a heroin addict who needs a fix into peaceful slumber is a bit of sexual stimulation administered by an obliging maternal hand.

Without incest, drugs and other clumsily introduced "problems" -- without the very things that are expected to sell this film as a shocker -- "Luna" might have made a charming comedy as the story of a high-spirited opera star sweeping through Italy with her adolescent son in tow. She is seen as a bubbly, creative, warm and exuberant artist, leading a crammed life, and the juxtaposition with the conventionality of an American teenager would be funny.

There are moments when it is. Her wailing look when he can't summon the interest to worship with her at the house of Verdi has good, wry humor.

But the big bad scenes require that we believe that this person has failed as a mother.

How? We see that she adores her son, from his babyhood, in maternal fashion, while her wifely love is focused on her husband, whose death deeply affects her. She works hard at an art she adores, collects good friends and admirers and is a success at everything. The only psychological crime that the film can throw at her is having a short attention span -- she does not focus all of her attention, all of the time, on her son. (And it doesn't seem long ago that that was considered the major maternal crime.)

There is one suggestion that incest is hereditary, and an unpleasant focusing on the monthly reappearance of the moon in the sky as boding some kind of evil. But chiefly, it is Mother who, no matter what she does, is held responsible for having turned an innocent baby into a morose junkie. Only a soap-opera mother would accept this.