With a meager ratio of jokes-to-laughs, “Due Date” winds up being only half a funny movie. Unfortunately, Robert Downey Jr. and Zach Galifianakis run out of amusing material before they get halfway from Atlanta to Los Angeles by car.

Downey plays Peter, a tightly wrapped architect whose wife is back in California, about to give birth to their first child. Galifianakis is Ethan, the Hollywood-bound would-be actor he meets curbside at the Atlanta airport – and winds up sitting in front of on the airplane.

But not for long: Ethan cluelessly gets them both kicked off the plane and placed on a no-fly list. Peter reluctantly accepts a ride from Ethan, who has rented a Subaru to drive himself and his French bulldog Sunny to L.A.

If you’ve seen the commercials for “Due Date,” you’ve seen most of the film’s funniest moments. Otherwise, you have to take what small pleasures you can from Downey’s slow-burn and Galifianakis’ bizarrely fey affect and random comments.

Downey is an actor who is constitutionally incapable of being uninteresting. The same is true of the delightfully subversive Galifianakis: He’s often funnier than the lines he’s given. But both of them can only do so much with a script that runs out of gags by the time they’re in Texas.

Downey and Galifianakis work it hard – but there’s just not much for them to work with in “Due Date.”

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