The Plod Couple: An inspired pairing in Due Date quickly resorts to cheap laughs.

You’re either going to love or loathe Zach Galifianakis when he puts himself into Todd Phillips’ hands. In The Hangover, Galifianakis’ borderline-pedophiliac, mentally challenged man-child was annoying enough to sink the movie. (Yes, the most successful R-rated comedy of all time does have some detractors.) And in Due Date, Galifianakis plays a gentler version of more of the same. His Ethan Tremblay is a 23-year-old (!) aspiring actor with a pocket pooch and the demeanor of a six-year-old kid. This time, at least, he’s occasionally funny instead of just irritating. In Phillips’ follow-up to his box-office smash, Ethan’s the thorn in an expectant father’s side from the time they meet disastrous at an airport. Peter Highman (Robert Downey Jr.) is trying to get back to Los Angeles in time for the birth of his first child. Ethan, meanwhile, is trekking to “Hollywood” to pursue his ain’t-gonna-happen dream. A car accident, bag mix-up, and light tussle on the plane gets both of them on the no-fly list, and with Peter’s wallet buried in his confiscated luggage, he has little choice but to accept Ethan’s offer to drive cross-country in his rented car. Near homicide, naturally, ensues. Four scripters’ efforts (including one responsible for the horrendous Made of Honor) result in a tone that’s all over the place: There are light Odd Couple-esque squabbles, dark humor, and, when all original ideas seem to have been exhausted about halfway through the film, strenuous wackiness à la car crashes and ridiculous schemes. Galifianakis is more likable here than he was in his breakout Hangover role—Ethan’s insistence on rules, such as not letting anyone touch his luggage in an airport, is mildly amusing—but it’s Downey’s wry, tightly wound, foul-mouthed bitterness that earns the best and most surprising laughs. (Bet you never thought you’d giggle at a grown man punching a kid in the gut.) Running jokes such as a coffee-can urn and that damn dog lose their bite quickly, however, as does the film in the aggregate. What starts out as an inspired coupling turns into another hate-you-love-you buddy flick, a moratorium on which is long overdue.