'Starman': a gentle sci-fi fantasy stressing human relations. Also, Jack Lemmon stars in 'Mass Appeal'

John Carpenter has specialized in the fantastic ever since he launched his career with ''Dark Star,'' a wry science-fiction comedy, and ''Halloween,'' which ushered in a new era of grim horror yarns.

His new picture, ''Starman,'' is a love story as well as a fantasy - a romance in more ways than one. The gentlest of his movies, it shows a new maturity in Mr. Carpenter's outlook, emphasizing close human relations rather than shocks and outlandish effects. Although it never quite comes together, it shows a shift of focus and interest that bodes well for his work to come.

The basic idea is an appealing one: Some civilization in deep space notices a greeting carried toward the stars by an American rocket and sends a scout to check out this primitive Earth and its inhabitants.

The emissary crash-lands in Wisconsin, which is a pretty place but is also 2, 000 miles from where his people will pick him up in three days. So he assumes a human shape by pilfering some DNA from a lock of hair in a family album. Then, since he doesn't know how to drive, he coerces a human being into transporting him.

And here's where the love angle comes in: It so happens that his reluctant chauffeur is a recently widowed woman, and his new form is identical to her late husband. She can't decide whether to adore or hate this familiar yet unearthly creature, until she gets to know him for himself and learns that spacepersons need affection, too.

The best parts of ''Starman'' are the most lighthearted. It's fun watching an amiable alien being figure out Earth's irrational ways, and when he stumbles on something sublime - like apple pie with whipped cream - his enthusiasm is both touching and hilarious.

Jeff Bridges adds to the humor with his eccentric portrayal of the hero, who never gets used to an earthly body and moves like some overage Pinocchio forever on the verge of becoming a real boy. Karen Allen is a good foil for his physical jokes and matches his emotions in the bittersweet love scenes that arrive in the second half of the picture.

''Starman'' might have been a first-class fantasy if its screenplay (by Bruce A. Evans and Raynold Gideon) were more clearly aimed. The action veers from science-fiction gimmickry and romantic dalliance to car-chase heroics and road-movie meandering, dipping into one genre after another instead of making up its mind as to which one it wants to belong to. The subplots and supporting characters are trite (government agents tracking down the alien), and echoes of ''E. T. the Extra-Terrestrial'' are too strong for comfort, even though the ''Starman'' story is much more grown up in tone.

I hope Carpenter brings the sensitivity of ''Starman'' to future projects, combining it with tighter screenplays and more original visual ideas. It's good to see the talented but often frivolous director of ''The Fog'' and the remake of ''The Thing'' lift his sights a little. His future looks promising if he follows through on his new direction.

'Mass Appeal'

Jack Lemmon's screen performances have varied greatly in recent years, from the crest of ''Missing'' to the trough of ''Tribute.'' His new movie, ''Mass Appeal,'' finds him pulling out every ingratiating gimmick in his repertoire - teetering on the brink of excess, yet never quite plunging into the hammy depths below.

It's ironic that Mr. Lemmon strains hard so to please us, because that's what his character in ''Mass Appeal'' does with his audience, too. He's a Roman Catholic priest whose glib, outgoing personality has landed him in a smug, wealthy parish, where he regales his flock with just the cozy, congratulatory words they want to hear in their weekly sermons.

But a challenge awaits him - in the form of a young seminarian who likes tweaking orthodox noses with radical talk about female priests and such. He's assigned to work at our hero's church, and the priest grows to respect his idealism, unchanneled though it is. When questions arise about the young man's past, including a busy period of sowing wild oats, the protagonist faces a hard choice: abandon his protege or risk the wrath of his conservative congregation (and superiors) by defending him.

Although the setting is denominational, the issues explored in ''Mass Appeal'' are universally human ones like loyalty, affection, the meaning of maturity, and the capacity for regeneration. Everyone in the movie wants to do the right thing - even the pompous higher-up played by Charles Durning - and the measure of a good clergyman is seen as a simple desire to help others.

But these appealing qualities don't compensate for the stagy structure of ''Mass Appeal,'' adapted by Bill C. Davis from his Broadway play, or for the trite remarks of trite characters like the devoted housekeeper and the reactionary pedant. While I applaud the movie's compassion, and commend its aspirations to literacy - a virtue in short supply on the Hollywood scene - I can't muster more than a ho-hum for its overall impact. Glenn Jordan was the director.