"Mr. Mom," now at area theaters, has its share of bright lines and funny moments, but if you bring anything beyond trifling expectations to this role-reversal farce, starring Michael Keaton and Teri Garr as a couple obliged to switch homemaking and breadwinning duties, it will be difficult to avoid feeling shortchanged.

Every domestic comedy series in the history of radio and television has probably exploited the situation that comes up for inadequate renewal in "Mr. Mom." Over the years, quite a few feature comedies have also had a go at the perceived hilarity of family men struggling to cope with housekeeping and child-raising when circumstances drive them temporarily out of the work force and their wives into it.

Keaton's character, a furloughed automotive engineer named Jack Butler, doesn't represent a significant improvement, intellectually or humorously, over Arthur Lake's Dagwood Bumstead when he was compelled by war or other crises to make do without Penny Singleton's gainfully employed Blondie.

John Hughes' screenplay sets up a potentially effective parallel situation that is then undermined by scatterbrained execution. It's impossible to judge whether the breakdown originates in the writing or whether it evolved out of Stan Dragoti's cloddish direction.

The Butlers' crisis isn't angled for neo-feminist approbation, since Jack and Caroline are introduced as a contented traditional couple; evidently, he finds his job satisfying and she's not clamoring for "liberation" from the home and their trio of kids--boys about 7 and 4 years old and a girl barely into the toddler stage. Once Jack is laid off and they both look for work, Caroline turns out to be more employable, so she resumes her interrupted career in advertising while Jack tries to fill the gap at home, awaiting a recovery in the automobile industry. On the other hand, it would have been preferable if the filmmakers started with the assumption that housekeeping and wage-earning roles may no longer be strictly divisible along sexual lines in the prevailing economy and culture.

The promising structural aspect of the plot is that episodes illustrating Jack's new domestic obligations and Caroline's new career obligations could be crisply and perceptively interwoven. Unfortunately, neither environment seems to have inspired much in the way of intelligent observation or clever fabrication. On the contrary, the vicissitudes of both husband and wife shrink to anything-for-a-chortle insignificance.

For example, when it suits their facetious convenience, the filmmakers overlook the fact that they've identified the Butlers as highly educated people. Jack, who designs automobiles, is mysteriously ignorant about wiring and the operation of the family appliances. The gamble is that the cartoonish spectacle of an overloaded washing machine going haywire will obscure the illogic of suddenly portraying your hero as an old-fashioned stupe of a Dad-ums. But are such hackneyed sight gags worth the sacrifice in character credibility?

There's no compelling reason to believe that Jack and Caroline couldn't cope with the switch forced upon them. The strains on their ingenuity and marital bond derive solely from the filmmakers' need to generate yucks. Dragoti and Hughes seem incapable of exploiting the pretext for the more realistic and knowing comic conflicts that would be appropriate to an allegedly smart, capable, modern couple.

Caroline's sneaky boss, played by Martin Mull, is obviously destined to make a play for her, but this complication is as unlikely to compromise her as that damned washing machine is to baffle Jack. All the crises are trumped up, and most of them remain oblivious to fertile areas of comic exploration. For example, apart from an interlude of confusion about school traffic patterns, Jack is never drawn into the culture of his oldest son's grammar school in the slightest way. Wouldn't Caroline, as one of the dwindling group of mothers available during the day, have handed him a potentially amusing burden of PTA activities along with the domestic chores?

The list of similarly neglected possibilities, on both the home and work fronts, could keep a halfway intelligent sitcom going indefinitely. "Mr. Mom" is starved for real-life comedy.

"Night Shift" launched Michael Keaton as a remarkably original, spontaneous comic discovery. "Mr. Mom" slows him down but doesn't cancel him out. It's encouraging to see Keaton attempt a change-of-pace identity right away, going from the self-centered, big-city wacko in "Night Shift" to a respectable middle American suburbanite. Unfortunately, Keaton seems to lack the writing and directing support he enjoyed in his debut vehicle.

It's obviously a waste of dynamic resources to put Keaton in an immobilized position. He's got such a crafty, irresistibly funny delivery that any number of potentially routine remarks may be unexpectedly enhanced. For example, Jack's reassuring farewell to Caroline when she leaves for her first day on the new job--"We got it covered, right guys?"--and his preamble to an argument--"All right, let's get into it"--have a slangy insouciance and emphatic silliness that are verbally Keaton's own and a delight to overhear.

It's a pity that "Mr. Mom" doesn't have enough of an idea of how to use a new comic star, better equipped than anyone since Jack Lemmon to embody aspiring young men with big ideas.