Sometimes, a film can have all the right ingredients: A proven, smart director, two immensely likeable stars, and a tried and true premise. And sometimes, even with all those ingredients, a film can rub you the wrong way or just fail. Unfortunately, “Due Date” is a perfect example of this.

Robert Downey Jr. and Zach Galifianakis star as two men forced to drive across the country together after a misunderstanding on a plane lands them on the no-fly list. However, time is short, because Peter’s (Robert Downey Jr.) first child is being born in a matter of days. The set-up is bound to draw comparisons to the John Hughes classic “Planes, Trains and Automobiles,” but where that film was heartfelt and warm in its humor, “Due Date” is obnoxious and annoying.

Almost every negative aspect to the film can be tied back to the screenplay, which is frustratingly inconsistent. It’s dark, but never enough to be taken seriously, and it never quite reaches the heights it’s aiming for in that respect. It’s got a handful of funny jokes, but that’s because of the sheer, irrepressible likability of its stars. Unfortunately, this about all the film has going for it. Its characters are empty vehicles, going wherever the lazy, obvious jokes demand they must. They don’t behave like real people, even though the film desperately wants us to take them and their arcs seriously.

Downey Jr. manages to come out mostly unscathed thanks to a few great moments, especially a late-night conversation at a rest stop that actually manages to be legitimately sweet. It feels like a scene out of a smarter, funnier film. Galifianakis, on the other hand, after his already-iconic role in last year’s “The Hangover” (from the same director), is ridiculous here, less a character than a collection of eccentricities and quirks played for laughs. It’s as if someone took his character from “The Hangover” and told him to be even weirder, but lose everything that made that character stand out in the first place. The results are almost depressing in their hollowness.

“Due Date” should have been a much funnier film, and on paper, it sounds like a surefire winner. However, an insurmountably weak script undoes the entire thing, despite the best efforts of its stars.