"Best Friends," opening tomorrow at area theaters, turns out to be exceptionally authentic and endearing--the most original and keenly observant romantic comedy to emerge from Hollywood since the underrated "All Night Long."

The results of this promising get-together of Burt Reynolds and Goldie Hawn are far more impressive than anticipated. But the primary credit for the film's ingratiating sort of comic realism no doubt belongs to screenwriters Barry Levinson and Valerie Curtin, who evidently distilled this hilariously accurate account of the perils of newly wedded life out of a traumatic chapter of their own relationship.

Levinson, who began his career as a comedian, made a successful debut as a writer-director earlier this year on the astutely observed '50s social comedy "Diner"; Curtin, a genius at playing neurotic character comedy, has contributed memorable performances to "Alice Doesn't Live Here Anymore" (as the jittery waitress Vera) and "A Different Story" (as Meg Foster's melancholy girlfriend) and adds a delightful unbilled bit to "Best Friends" as the pal Goldie Hawn gets giddily potted with while discussing her nagging doubts about marriage.

The Levinson-Curtin collaboration ripens in a particularly savory form in "Best Friends," which suggests that they may be evolving into the sharpest, subtlest marital comedy writing team since Garson Kanin and Ruth Gordon in their prime.

At any rate, they discover a fresh comic approach to the traditional "period of adjustment" situation confronting a newly married couple. Their fictional alter egos--Reynolds and Hawn as a successful pair of screenwriting lovers named Richard Babson and Paula McCullen, who have been working together productively for five years and living together compatibly for three years--are the sort of modern, career-centered people who set themselves up for slightly newfangled falls by edging into marriage relatively late.

There's no attempt to pass the stars off as younger than their real ages. Richard is a bachelor in his mid-forties and Paula ditto in her mid-thirties. Since they're scarcely kids and share the same domicile already, they can't envision the marriage ceremony as a joyously symbolic rite or public occasion. So they keep it quiet and blunder awkwardly through a haphazard no-frills service at a Mexican-American chapel where the justice of the peace, Richard Libertini, struggles to finesse a broken English that causes unforgettable phonetic confusion, especially over the repetition of the phrase "I thee endow."

Leaving a movie-in-progress in the hands of a devious, absent-minded, narcissistic young producer (Ron Silver in a sensational caricature of a New Breed talent manipulator, at once batty but irresistibly determined when it comes to protecting his self-interest), Richard and Paula decide to pay courtesy calls on their parents, who live on the East Coast and have never met their new children-in-law. Unfortunately, this family-mollifying honeymoon turns into a sustained, richly recognizable comic disaster. Boarding a cross-country Amtrak in Los Angeles in February, Richard and Paula share three days of exhausting, uncomfortable close quarters and nagging inconveniences, only to arrive in frigid Buffalo, the home of Paula's parents. Played by Jessica Tandy and Barnard Hughes, the McMullens are now so old and pestered by various infirmities or senilities that the already shaky newlyweds are inevitably reminded of the deteriorating stages of life and marriage.

A brilliantly realized sequence, the stopover in wintry Buffalo proves a classic of pinpoint social comedy from the opening wacky touches, when Dad McMullen welcomes Paula's gift of California oranges with the cheerful observation, "The ones from Florida are juicier," and Ma McMullen, wearing dark glasses in the wake of a cataract operation, tells the shivering Richard, "You should see Buffalo when everything is frozen over."

Levinson and Curtin must have minted this material from real life, because it keeps reproducing the authentic, mind-boggling obliviousness that characterizes ordinary social intercourse, especially within family groups. It's intensified in this instance by the advanced ages of the McMullens, whose incorrigible tendencies are also far advanced, but the movie's view of their marriage is essentially tender and affectionate. Moreover, the film captures other telling insights -- the way one's behavior tends to regress when returning to a childhood home, for example, and the way the mate who feels at home tends to be complacently amused by the discomfort of the mate who's a total stranger to the family.

In Buffalo, of course, it's Richard who does most of the squirming. When the scene shifts to McLean, where Richard's parents, played by Audra Lindley and Keenan Wynn, are living out a comfortable retirement in the towering Rotunda condo, Paula is on the defensive. The rift widens between the newlyweds in this contrasting in-law environment, and grows even wider because Richard and Paula are self-conscious, psychologically wised-up people, inclined to resent it even more when the other falls short of emotionally supportive expectations, spoken or unspoken. Paula, already miffed at Richard for failing to announce their marriage to his family before they arrived, is so tense with her in-laws that Richard encourages her to relax with an occasional Valium. When she overdoes it and relaxes herself into a swoon while shopping with Richard's mother and sister, Richard can't help feeling personally betrayed too -- it's as if she'd said that only a coma could make the society of his family tolerable.

"Best Friends" profits from both a sounder structure and a finer quality of observation than movie comedies usually exhibit these days. Nevertheless, it seems to play with a pleasant illusion of spontaneity and looseness. Levinson and Curtin don't ignore comic payoffs any more than Neil Simon would, but they're able to conceal the manipulations that generate the laughs in a texture of authentic character eccentricity and social interplay.

Norman Jewison, who emphasized wretched excess when directing their ". . . And Justice For All" script, responds with an admirably dry, incisive presentation of this vastly more coherent, respectable and appealing material.

If anything, the stars seem to be unusually generous foils for the supporting players, handling the straight side of the exchanges while Silver, Tandy, Hughes, Curtin and the others collect the comic benefits. Individually, I suppose Reynolds and Hawn have certainly enjoyed showier showcasing, but it should do them no harm at all to be recognized as a likably self-effacing romantic comedy team in a new romantic comedy of rare sweetness and intelligence.