Todd Phillips' wildly uneven career as a comedy director takes another dive with "Due Date," an oddly somber "Planes, Trains and Automobiles" rehash that cannot be saved even by the combined quirky talents of Robert Downey Jr. and Zach Galifianakis. They are only human, after all.

This will be a disappointment for viewers hoping for a repeat of last year's "The Hangover," a similarly raucous adventure that also co-starred Galifianakas and was directed by Phillips. You have to remember that for every "Hangover" and "Old School" on Phillips' resume, there's a "Road Trip" and "School for Scoundrels." "Due Date" is one of those.

Downey plays Peter Highman, a business-y sort of businessman guy who's in Atlanta on business and is heading home to L.A., where his wife (Michelle Monaghan) is about to give birth. But thanks to some implausible and maddeningly unfunny hijinks perpetrated by Ethan Tremblay (Galifianakis), a dumb, vaguely swishy would-be actor on Peter's flight, both men are kicked off and put on the no-fly list. Having lost his wallet and with no other way to get home, Peter hitches a ride with Ethan Tremblay, who has rented a car.

You can well imagine the sort of shenanigans this odd couple gets in to on their cross-country trip! Peter is stiff, tightly wound, somewhat angry; Ethan is laid-back and carefree, sweating neither the small stuff nor the big stuff. "How have you survived?!" Peter asks him. "Mostly luck," Ethan says. He is messy and ruins things! He falls down sometimes! Thus begins the Chris Farley-ization of Zach Galifianakis, somewhat earlier than scheduled.

What is especially peculiar about this alleged comedy -- written by "King of the Hill" scribes Alan R. Cohen and Alan Freedland, with rewrites by Adam Sztykiel ("Made of Honor") and Phillips -- is how frequently it stops trying to be funny altogether and wants us to have actual feelings for these comic-strip characters. Ethan is mourning the recent death of his father and has a coffee can full of his ashes, while Peter has daddy issues of his own. Sometimes Ethan and Peter have moments that aren't just not funny -- they're actually serious. It's ineffective, though, because the film's whole premise is silly. We're only going along with it in the hopes of finding surface-level hilarity. If anything other than hilarity is on the menu, we're not interested.

(Phillips makes some weirdly downbeat choices for the soundtrack, too, including contemplative numbers by Neil Young, Band of Horses, and Fleet Foxes. Seriously.)

Not that the film is entirely devoid of laughs, of course. Downey and Galifianakis have good comedic chemistry that occasionally hints at what could happen if they were given a better script. (I suspect the outtakes and ad-libs are funnier than what wound up on the screen.) What I noticed, though, is that while there are individual moments that earn a chuckle, there are no entire scenes that work. The laughs are sporadic at best, and downright scarce in the second half, as the film trudges from one mishap to the next.

Grade: C

Rated R, a lot of harsh profanity, some strong sexual dialogue

1 hr., 40 min.

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