You know what? Sometimes, two great tastes don't go great together.

Gin plus tequila equals immediate emetic. Surfing's cool. Giant sharks are very cool. But try teaching a giant shark to surf and you'll need a drink so bad you'll take a Tanqueray margarita in a heartbeat. Likewise, you should not walk on stilts among a field of adorable bunnies.

-- Local show times, theaters, trailer.

Casting Robert Downey, Jr. and Zach Galifianakis — two of the funniest people alive — as the leads in a comedy sounds like an epic win so winful it could win against the New York Yankees, who field a Godzilla or a Frankenstein at every position. For Due Date, it only sort of works.

Downey, Jr. is on schedule to fly home in time for the birth of his first kid — until a boarding incident with fellow passenger Galifianakis gets him kicked off the flight and placed on the no-fly list.

Downey's wallet and driver's license get left on the plane, however. Unable to rent a car to get home from Atlanta, he's forced to accept a cross-country ride from Galifianakis, an obnoxious would-be actor who seems to foul up everything he touches.

Road trip stories are by nature episodic. Anything you do or anyone you meet is swiftly left behind at 70 mph to be replaced by the next setback or shenanigan. Of course, this isn't true of classic caveman road movies where the characters can spend a full week walking through the same flock of hostile pterodactyls, but when it comes to the modern era, you're not going to find a whole ton of dramatic continuity.

This is especially true of Due Date, which is often little more than a feature-length collection of alternating scenes where a) Galifianakis does something to piss Downey off and b) circumstances conspire to force them to continue traveling together. This isn't entirely a complaint — like I said, it's part of the genre — but Due Date doesn't help out its case with its plot.

At least its "And then we went here, and found more trouble" structure is populated by dependably funny bit players like Danny McBride, Juliette Lewis, and RZA. They frequently serve as humorously offensive people for Downey to beat up (Lewis' bratty son) or get beat up by (McBride as a paraplegic veteran).

There's a fair chunk of that There's Something About Mary-style shock humor in Due Date, and your appreciation for such stuff will go a long way towards your total laugh count. Otherwise, you'll likely be kinda lukewarm to it as a comedy.

Galifianakis' devotion to his effete, clueless, stoned incompetent can be pretty great, but as a hotheaded, put-upon dude with a hot wife and a nice job, Downey's not as funny as normal. Sure, that's like saying the fire I fell into last week didn't hurt as bad as the one I tumbled into in '08, but when you're used to skin grafts the size of circus tents and this time you only end up needing some bandages and aloe — wait, I'm almost to my point — it's hard not to be let down.

There may also be some "unpleasant next day-ish feelings" effect following up director Todd Phillips' The Hangover. But while Due Date is likable and funny enough, its four-man writing team is sometimes at odds with itself tonally, and between the bunch they couldn't drum up a compelling plot. Nor do they quite hit the heart they're aiming for. It's easy to watch. It may not be so easy to remember a few weeks from now.

Grade: B-