Director Todd Phillips has done some pretty funny films, including “Old School” and last summer’s instant classic “The Hangover.” Both of those films combined a funny script and a great cast. Unfortunately, with his new film, “Due Date,” Phillips only got the formula half right.

Peter Highman (Downey, Jr) is on his way home to Los Angeles via Hartfield Airport in Atlanta. His wife Sarah (Michelle Monaghan) is scheduled to give birth shortly and he plans to be present during the delivery. As he arrives he has an unexpected run-in with Ethan Tremblay (Galifianakis, looking spookily like Joaquin Phoenix after his goofy appearance on “Letterman” ), who is unwittingly heading for the same plane. Mix ups prior and after boarding leave both men grounded with no way to get out west except by rental car. Let the adventure begin!

Let me say right off that I liked this movie the first time I saw it, when it was called “Planes, Trains and Automobiles.” And while “Due Date” features a fine duo of actors if doesn’t have the sweetness and laughs of the John Hughes film. But this is through no fault of the cast. Downey has beaten almost every demon imaginable to become one of the finest actors of his generation. He is strong here but even though Peter is stuck in a situation not of his making, you don’t really feel sympathy for him because he’ s such a jerk. I’m not saying that I wouldn’t react the same way he does in the same situation, but that doesn’t mind I would like myself afterwards. As Ethan, an actor-wannabe who runs a fan-site called “It’s Raining Two and a Half Men” and longs to appear on the show, Galifianakis brings the same sweet quality he showed in “The Hangover” and combines it with some surprisingly strong dramatic chops. Traveling cross country with his late father’s ashes in a coffee can and his dog, Sonny (a scene stealer), you feel more for his plight in the adventure then you do Peters. This is in part to a script that may have “read” funny but plays out as a series of one unbelievable moment after another. Granted, many of them are mildly funny, but they seem to have been written by a group (the film is credited to four writers) who kept yelling out, “Oh, do you know what would be really funny?” during the writing process. Two of the writers came from television, where “do you know what would be really funny” works in 22 minute episodes, and have no feature screenplay experience while another’s only script was the unfunny “Made of Honor,” which itself was a rip-off of “My Best Friend’ s Wedding.” Note to future script writers: there is nothing wrong with originality.

The supporting cast is pretty much wasted, with only Danny McBride delivering on the funny. Jamie Foxx appears to be here only because he’s Jamie Foxx, and his character is really nothing more than a red herring in the story. Juliette Lewis and RZA complete the roster of “names” in small roles, but since they’re not writers they don’t really help much. If there is any bright spot, it’s that Phillips and Galifianakis are now hard at work on “ The Hangover 2” while Downey is filming another Sherlock Holmes film. Hopefully 2011 will be better for both them and us.

On a scale of zero to five stars I give “Due Date”