Rarely is a director as ambivalent about what kind of movie he wants to make as Chris Columbus is about Nine Months, the latest film to capitalize on Hugh Grant's golden grin. Columbus made a name for himself in the slapstick and hijinks genre with his Home Alone movies, and he managed to attract an adult audience with 1993's Mrs. Doubtfire. The transition from a Macaulay Culkin audience to the Robin Williams crowd worked because he didn't have to change the formula much; the difference between knocked-cold bad guys and flaming falsies is really one of degree more than of substance, and parents who took their kids to see Mrs. Doubtfire were simply grateful to escape watching Culkin slap his cheeks.

With Nine Months, Columbus looks to move one step closer to making grown-up movies. The problem with this latest effort, though, is that it aspires to be a serious film -- with, like, themes and stuff -- yet it can't let up on the madcap shenanigans long enough to say anything. Columbus has failed to recognize that the soda pop of light physical comedy -- even if it's really good soda pop -- doesn't mix well with the Port wine of the romantic tearjerker. The concoction he comes up with isn't too tasty, and it doesn't get you drunk, either.

Grant plays Samuel Faulkner, a child psychologist whose palms sweat every time his longtime girlfriend Rebecca (Julianne Moore) mentions "the big M." When Rebecca tells him that she's pregnant, and that he'd better decide quick whether he's a stand-up kind of guy or not, he doesn't take it too well. He starts dreaming he's a praying mantis, about to be devoured. And in his panic, he loses the one thing in his life that means anything to him: his sports car -- I mean, Rebecca; he loses Rebecca. Then he gets his priorities straight, and then he loses the sports car.

Columbus, who also wrote the script, has put his hero in a nearly impossible spot: he likes his life the way it is. It's hard to fault him for that. Every child Samuel sees is impossibly odious, and his friend Sean (a typically deadpan Jeff Goldblum) has filled his head with visions of voodoo women, "putting pins in their diaphragms." Worse still, Samuel has to contend with the near-constant antagonism of philistine car salesman Marty Dwyer (Tom Arnold) and his family. The Dwyers, as characters, walk that thin line between annoying the other characters and annoying the audience -- a tightrope that Tom Arnold has fallen off of more than once. The Dwyers are meant to sweep Rebecca, and ultimately Samuel, up into the fervor of childbearing. But one look at the children is enough to make anyone hit their biological snooze button.

Much of the film is consumed by fights between Marty and Samuel: fisticuffs that seem to belong more to the Keystone Kops oeuvre than to the realm of romantic comedy. And when they aren't fighting, some other bit of physical comedy emerges, usually involving pain. Nine Months boasts 23 stunt performers, quite a crowd for a movie the main conflict of which is internal. Not only does the roughhousing fall short as slapstick, it generally manages to disrupt whatever momentum the film has mustered. Only toward the end does Nine Months succeed even as a tearjerker, and that only lasts until Grant and Arnold go tumbling over a delivery-room table.

The most surprising disappointment of Nine Months is Robin Williams's failure to cull out a few bright spots as the inept substitute obstetrician, just in from Russia. Dr. Kosevich's joke is that he doesn't speak the language too well, and so he often uses the wrong word: "clitoris" for "thesaurus," for example. Some joke. It's a shame to see Williams's comic talents wasted on salacious malapropism, and he isn't on screen long enough to contribute much else.

As the eagerly expectant mother, Julianne Moore manages to keep some dignity and depth about her during the proceedings. She's usually on the screen when Nine Months takes its occasional turn for the better. Of course, Moore has the luxury of being otherwise occupied when most of the mayhem breaks loose; when she is in the scene, she tends to get busy fast. Grant is less fortunate. He must contend with the rigors of epiphany, with only a winsome grin and a few pratfalls at his command. He doesn't perform badly, under the circumstances. In fact, he and Moore manage a certain chemistry whenever they're left alone long enough to work on it. Columbus is so in love with his sight-gags, though, that they're left trying to blow soap bubbles on top of a speeding train.

In all fairness to the writer/director, the audience members who didn't care about the dramatic aspects of the film -- about a third of them -- laughed almost continuously. Columbus has already proven himself capable of roundhouse comedy, and for all its flaws, Nine Months hints that he may have a more substantive story to tell. I hope that, with his next film, he either sticks to what he does well, or has the courage to leave it behind. Nine Months makes it clear that he can't have it both ways.