THE special providence that protects movie critics decrees that when they do take up honest work they often make surprisingly good movies. Godard and Truffaut, come to mind at once, but also a whole line of "Cahiers du Cinèma" critics including Chabrol, Rivette, and Eric Rohmer.

In America, we have Peter Bogdanovitch ("Targets") and now Susan Sontag with "Duet for Cannibals," which played last night at the New York Film Festival. Miss Sontag's credits extend, of course, a considerable distance beyond movie criticism, but she has been one of the best of critics, and I have heard some of her colleagues remark, with disarming generosity, that she has proven herself so good at making movies you'd never guess she had written about them.

Except for some bandages out of Godard, two wigs out of Antonioni, and a leading lady out of Bernardo Betrolucci (Adriana Asti, who is more interesting here than she was in "Before the Revolution"), "Duet for Cannibals" doesn't seem to owe much to anybody except to Miss Sontag and her own idomatic, uncluttered sense of the medium.

The film is in Swedish, made in Sweden for a Swedish producer, but the subtitles are Miss Sontag's, and I suspect that as much has been gained as lost in the various translations and transpositions required in realizing the project.

The cannibals are a middle-aged radical German political activist and the theoretician, Bauer—Hans Erborg—living with his young Italian wife Francesca—Miss Asti—in Sweden. Their victims are a young Swede who goes to work as Bauer's secretary, and his mistress, who eventually finds herself working as the Bauers's cook and companion. For all the movie tells us, Bauer's credentials are real enough (down to a chrome-plated cigarette lighter—gift of Bertolt Brecht), but everything in his present life partakes of fraud calculated to intrigue, upset, and entrap his assistant.

His erratic and violent behavior, the temptation palpably and leeringly offered of his beautiful young wife, eventually the intellectual challenge of what move he will make next, engage the young man and put him repeatedly off balance.

Before it is all over the girl is at work too, making love to the master, accepting advances from the mistress, feeding and being fed by both of them, and lying between them in their connubial bed.

There are too many insane people in the world, comments the young hero after he is attacked by a madman on a city street, and of course he includes the Bauers, who also attack—and win—because they try anything and stand by nothing.

Nevertheless, I don't think "Duet for Cannibals" means to be a parable about the power of the insane over the sane, or the strong over the weak, or even the inventively absurd over the rational and passionate. I don't know what it does mean to be, and I am content for a while to rest with its moods and its complicated, often funny motions.

But if the movie fails—as I think it does—to open up beyond the strength and the tact of its specific scenes, it invites that failure in the limitations of its own point of view and in its insistence on insoluble mystery to the point where mystery grows boring without getting less mysterious.

The young couple's final escape offrs relief of a rather low level—mostly that the charade is over for them and us. The personal games increase in intensity, but nothing very much is at stake, and personality is never deeper than the next level of plausible disguise.

"Duet for Cannibals" will be shown again at Alice Tully Hall on Friday at 6:30 P.M.

DUET FOR CANNIBALS, screenplay by Susan Sontag; directed by Miss Sontag; produced by Goran Lindgren; production company, Sandrews, distribution in the United States by Grove Press, Inc. At the New York Film Festival, Alice Tully Hall, Lincoln Center. Running time: 105 minutes.

Francesca . . . . . Adriana Asti
Bauer . . . . . Lars Ekborg
Tomas . . . . . Gosta Ekman
Ingrid . . . . . Agneta Ekmanner