A bracing new comedy sleeper, "Night Shift," is poised to duplicate the successful pattern created last summer by "Arthur." It figures to open modestly this weekend, then steadily build a following as news of its undeniable likability and freshness gets around. Eventually, that following should turn it into one of the box-office sensations of the year.

The largest shares of credit for this pleasant surprise evidently belong to director Ron Howard--whose assurance behind the camera may come as a revelation to people still associating him with the roles of little Opie on "The Andy Griffith Show" and clean-cut Richie on "Happy Days"--and to co-stars Henry Winkler and Michael Keaton, who emerge as an inspired comedy team.

An inventive, good-natured elaboration of a slightly outrageous caper premise, "Night Shift" celebrates the benignly illegitimate, bootstrap co-op enterprise of two wimpy but resourceful misfits, Winkler's Chuck Lumley and Keaton's Billy Blazejowski, familiarly known (at his own request) as Billy Blaze, who form a mutually rehabilitating, liberating partnership while working as night attendants at the New York City morgue.

The timid Chuck is the senior employe. Accustomed to letting aggressive types walk all over him, Chuck has meekly accepted a transfer to night duty from a boss who's awarded the day job to an imbecilic nephew (a fleeting but hilarious role played to perfection by the cagey young actor Bobby DiCicco) Although the boss' reassurance that the night job is really more desirable--"There's no supervisors around, you practically run the place"--sounds dubious at the time, it proves serendipitously correct once Billy Blaze, the new night assistant, walks--or rather, barges, with a raucous, wacky flourish--into Chuck's life.

Keaton gets a brilliant entrance. We hear motor-mouth Billy, evidently imitating the instrumental of a rock recording, before we see him, and the comic suspense is sustained by having his bobbing silhouette loom outside Chuck's office door before he appears in the motley but contagiously funny flesh. Brash, garrulous and blithely irresponsible, Billy is a comic imp sent to pester nice, self-effacing Chuck out of his passive rut. "You wanna know why I carry a tape recorder?" Billy asks for openers and then instantly explains, "I'm an idea man--I get ideas all day long; I can't control them." He babbles brainstorms into the recorder as they occur to him and mentions, twice, that he came that close to inventing wash-and-drys--"I thought of it first, only they already had it."

At first Chuck perceives Billy as a noisome pestilence--the latest trial in an existence as bleak as Job's. However, he is too nice to resist the sincerity of Billy's offer of friendship, and eventually he allows himself to entertain the possibilities in one of Billy's harebrained notions, inspired by one of the stiffs who passes through their office--a murdered pimp. The body is identified by one of the victim's hookers, Belinda (Shelley Long), who soon renews acquaintances with Chuck when it turns out that they live in neighboring apartments. Finding her roughed up one night, he offers assistance and later repeats her hard-luck story to Billy--that the girls have been having a difficult time avoiding trouble since their pimp checked out.

A lightbulb explodes somewhere in Billy's overactive, demented imagination and he proposes that they take over the stable. What makes this ridiculous and possibly dangerous idea attractive to the sensible, scaredy-cat Chuck is that he's aroused by Belinda and finally motivated to defy, if only in secret, the principal emotional bullies in his life--a frustrating girlfriend played by Gina Hecht and a dreadnought mom played by Nita Talbot. Moreover, the scheme wouldn't work without him, because Billy couldn't possibly follow through on the practical implications of his idea. The partnership evolves, with sweet-tempered improbability, as an arrangement that pays off handsomely for the girls as well as the guys. Chuck uses his real training and skills to invest their wages prudently, functioning as a legitimate business manager instead of pimp.

Still, the racket remains prostitution (one of Keaton's funnier routines is a preposterous attempt to impress the girls by breaking down the etymology of this word), so it's only a matter of time before organized crime or the authorities interfere. By that time the filmmakers have everything deftly positioned for a happy getaway, and all the characters you've grown partial to are assured of more or less prosperous and respectable futures.

Several years ago Henry Winkler graciously accepted an Emmy for his performance as Fonzie in "Happy Days" by paying tribute to the generosity of the other cast members, notably Ron Howard. It wouldn't have been possible for Fonzie to walk on and steal the show, he pointed out, unless colleagues like Howard were willing accomplices in the theft. Perhaps the sweetest irony of their collaboration in "Night Shift" is that Winkler should now emerge as an appealing romantic comedy lead by assuming the straight-man role that Howard used to fulfill for him on "Happy Days." The brilliantly antsy, slangy, spontaneous Keaton is entrusted with the forceful, scene-stealing comic business in "Night Shift." His Billy is the manic, agitating gadfly who causes things to happen while Winkler's Chuck provides the contrasting temperamental reserve and resistance.

Winkler is vastly better when he's not obliged to act the madcap, an identity that seems to come naturally to Keaton. Winkler seems funnier in "Night Shift" than in his previous feature vehicles because he isn't trying to bowl anyone over or dominate the proceedings. Wearing a demeanor reminiscent of Jack Benny and speaking like the real-life, gravely self-serious Woody Allen, Winkler creates an image of put-upon comic dignity and sensitivity that makes Chuck's well-deserved liberation a richly gratifying deliverance.

Keaton has been refining Billy's squinty-faced, screwball impishness on a number of television series that never quite clicked--"All's Fair," the Mary Tyler Moore variety series, "Working Stiffs," "Report to Murphy." If he has an unsold pilot or two still kicking around, he'll probably be praying that they stay unsold, because "Night Shift" ought to do as much for Keaton as "Animal House" and "Meatballs" did for John Belushi and Bill Murray, respectively.

What gives "Night Shift" decisive sparkle, of course, is the chemistry between Winkler and Keaton. One presumes it was Ron Howard who encouraged and guided this process. Although "Night Shift" is his first major feature credit, he prepped on three television movies and a Roger Corman quickie, "Grand Theft Auto," that returned a handsome profit on a low-budget investment. However, it's only natural to underrate the abilities of a young man who virtually grew up on your television set. "Night Shift" makes it apparent that Howard acquired an enormous amount of savvy about comic staging and acting during those 20 years or so on the tube.

There are occasional misjudgments--Howard repeats a few running gags that illustrate Chuck's timidity once too often, takes a few dead-end plot tangents and leaves a few potentially funny situations half-baked or overcooked. However, the prevailing level of comic craftsmanship is delightfully ingenious and astute.

Perhaps one great superfluous character detail will suffice to illustrate the comic virtues of this movie. Winkler makes Chuck obsessively orderly about his office desk, and watching him get everything in its right place is funny enough, but when he sharpens a pencil in an electric sharpener and then promptly empties the sharpener, you sense that this is a comedy with that little, indispensable, something extra.