'Falling in Love':deliberately flat story of suburbia.

Even before the credits are over, ''Falling in Love'' pitches the theory that married suburbanites are a faceless, indistinguishable breed. We see the main characters board the same train from identical stations in side-by-side towns. When they talk to their spouses on adjacent phones, their conversations mesh so perfectly they could be speaking with each other, even though they've never met. Later they shop at the same bookstore and chat with strikingly similar friends.

It's possible the movie throws in so many coincidences to suggest an element of fate when these two finally do meet, fall for each other, and spend a year working up the nerve for an affair. But the filmmakers show a subtle contempt for their characters by stressing life styles instead of lives. Frank and Molly aren't just made for each other, they're clones of each other, walled in by deadening symmetries of culture and convention. Ditto for their friends and spouses, who pass through the same petty experiences a few months earlier or later.

''Falling in Love'' has every right to this view of suburban existence, which is thought-provoking if gloomy, and I hoped the film would explore it intelligently. The screenplay is as inhibited as its characters, though - rarely staring a problem in the face or taking direct action, but dribbling its energy away in little dances of anticipation and anxiety.

The result is a safe, dull drama about safe, dull people: a ''Brief Encounter'' for commuters. True, it's refreshing to see a love story that holds back on sex, limiting itself to one unconsummated bedroom tryst. Yet the morality of ''Falling in Love'' isn't exactly pristine - the hero lets his marriage fall apart while mooning over his new girlfriend, the heroine loses interest in her husband, and it's implied that their affair will become fully sexual when those boring marriages are scrapped. Though some involved in the production have noted its restraint, it marks no advance over other pictures (''Tender Mercies'' and ''Local Hero,'' say) that took quieter but equally strong stands on sexual responsibility.

What the picture does have going for it are predictably good performances by Robert De Niro, still the best movie actor of his generation, and Meryl Streep, who has reached beyond mere craft in her recent films and discovered a new level of emotional expression. The supporting roles are capably handled by Dianne Wiest and Harvey Keitel, among others, when the sketchily written screenplay (by Michael Kristofer) gives them a meaningful moment now and then. 'Supergirl'

It's not a bird, it's not a plane, and it's not Superman, either. It's his teen-age cousin, sent to earth on a desperate mission to salvage some remnant of the Man of Steel's movie series - a feat even Richard Pryor couldn't pull off, although he tried in the last ''Superman'' picture.

''Supergirl'' begins on a faraway planet that seems to consist of one large room. Kara, the heroine, is fiddling with a little object called an ''omega hedron,'' which suddenly flies out the window.

This upsets everyone, since the doodad is a power source for the planet, and now everyone will die within a couple of days. Kara is very embarrassed - wouldn't you be? - and promises to retrieve it. This means going to Earth and fighting a sorceress who's bent on that old favorite, world domination.

The movie has two good gimmicks. One is its acting style, broad and campy even by ''Superman'' standards. The performers are having fun, and it's infectious. ''Supergirl'' also solves the problem of finding an effective villain - someone who can stand up to a Person of Steel and have some chance of winning. As the wicked witch of the story, Faye Dunaway calls on supernatural spooks that rattle even a visitor from ''inner space'' with superstrength and all the rest.

But such nonsense can't sustain itself for long, even with Dunaway huffing and puffing with enough energy for three sequels. The plot is too slim and illogical, and the characters are echoes of the ''Superman'' folks, especially when Dunaway and Brenda Vaccaro reprise the Abbott and Costello-style banter that Gene Hackman and Ned Beatty wore thin a few years ago.

Helen Slater deserves credit for playing the heroine with commendable conviction, though, and for making her secret identity (as a mousy schoolgirl) more convincing than the old Clark Kent routine ever was. Jeannot Szwarc, the auteur of ''Jaws 2,'' was the director.

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