‘Due Date’

We've seen "Planes, Trains and Automobiles." We know "Planes, Trains and Automobiles."

And "Due Date," you're no "Planes, Trains and Automobiles."

More's the pity, because the prospect of embarking on a comedic cross-country jaunt with Robert Downey Jr. and Zach Galifianakis sounds like it should be fun.

Unfortunately, it's not enough fun to make the cinematic trip worthwhile. At least not with "The Hangover's" writer-director, Todd Phillips, behind the wheel.

Like its top-grossing predecessor, "Due Date's" lewd, rude and crude -- but only sporadically so, leaving us hanging for long stretches, waiting for the crazy comedy to kick in.

Sometimes it does. More often than not, however, "Due Date" is crazy without being all that comedic, forcing us to sit through a cinematic road trip that feels almost as draggy (and sometimes as druggy) as the one shared by the movie's odd-couple traveling companions.

In this corner: uptight Peter Highman (Downey), a buttoned-down businessman who's headed home to L.A. from Atlanta so he can be with his wife (Michelle Monaghan) when she gives birth to their first child -- via a scheduled Caesarean section.

Due to circumstances way beyond his control -- and way beyond our credibility -- Peter finds himself booted from his flight home, thanks (or no thanks) to the walking-disaster presence of one Ethan Tremblay (Galifianakis).

An aspiring actor who's on his way to Hollywood -- inspired by the shining artistic example of TV's "Two and a Half Men" -- Ethan's as eager and overwrought as Peter is wary and reserved.

That's hardly surprising, considering the fact that Ethan's father has just died. (He's got the ashes in an empty coffee can to prove it.)

Barred from traveling by air, Peter and Ethan find themselves in the front seat of a rented Subaru. Ethan's French bulldog, Sonny, is in the back seat, on duty whenever crinkle-faced reaction shots and other assorted visual punch lines are required. (Which is too often for "Due Date's" own good.)

And off we go, barreling along the interstate with our misfit uneasy riders, who naturally make frequent stops for everything from Ethan's "glaucoma" (and the medical marijuana he uses to treat it) to close encounters with, among others, a surly Western Union agent (Danny McBride) and Peter's old college pal (Jamie Foxx).

Along the way, "Due Date" -- scripted by Phillips and Alan R. Cohen, Alan Freedland and Adam Sztykiel -- attempts to create laughs by exploiting the protagonists' comedic contrasts.

You know the drill: Ethan's a quivering (Tremblay-ing?) ball of emotional jelly, while Peter locks down his feelings, burying them beneath a thick layer of smug superiority.

We know that, eventually, these two will bridge the Grand Canyon-sized chasm separating them to find common ground.

At least "Due Date" doesn't go all touchy-feely on us as it slogs toward that inevitable destination. But it doesn't exactly take any unexpected detours, either.

Occasionally, the movie seems as though it's veering off into dark and edgy territory, daring to tiptoe toward scenes that might fully explore Peter's understandable hostility toward the maddening Ethan.

Every time that looms on the horizon, however, "Due Date" takes the safer, more obvious route toward gross-out jokes and heavy-duty slapstick.

Some of those sequences are genuinely amusing. But there aren't enough of them to give "Due Date" the kind of comedic drive it needs to maintain its momentum.

Instead, the movie lurches along in fits and starts, sputtering from one allegedly outrageous adventure to another.

Along the way, Downey and Galifianakis do their best to rev up the underpowered script, but the contrast between them isn't enough to keep "Due Date" in gear.

Galifianakis gives Ethan heaping helpings of idiosyncrasies, from his slovenly showbiz sartorial style to his say-anything, do-anything behavioral excesses. But we've seen it all before -- and it was funnier the first few times around. Or maybe it's because we've also seen Galifianakis' subtler, more sensitive side in "It's Kind of a Funny Story," and we know that he's capable of far more than "Due Date's" getting-stale shtick.

Downey has the opposite problem. He may be an infinitely resourceful actor, but even he needs something to play. And his deliberately underwritten, generic straight-man role doesn't leave him much room to maneuver beyond Peter's increasingly desperate aggravation -- and his inevitable, ultimate surrender to Ethan's inexorable tenacity.

Sooner or later, through sheer force of will, we know Ethan will wear Peter down.

Whether "Due Date" ultimately wears you down depends on your willingness to go along for the ride.

But it's not likely to wear you out with laughter.

Contact movie critic Carol Cling at ccling@reviewjournal.com or 702-383-0272.