Let’s face it: After bringing us the epically original “The Hangover,” director Todd Phillips has got some tough shoes to fill – his own. With its 2009 release, “friendships” quickly became known as “wolf packs” and a blundering bearded man was put on the map as a comical genius. As word broke that our beloved blunderer, Zach Galifianakis (“It’s Kind of a Funny Story”), was to unite not only with the guys who brought us the best blackout of our lives, but also the utterly charming Robert Downey Jr. (“Iron Man 2”), it seemed like the perfect onscreen duo was born.

Unfortunately, while the one-man wolf pack and Tony Stark might have seemed to compliment each other nicely, the characters each actor plays in “Due Date” are the worst shadows of their former characters, turning a potentially light-hearted road trip into an awkward and nerve-racking nightmare. “Due Date” is a fairly simple and often exhausted concept. Downey Jr. plays the part of Peter, an uptight and bad-tempered man who is forced to ride across the country with the eccentric Ethan Tremblay (Zach Galifianakis) in order to make it to his child’s birth. Ethan and Peter have five days to cross 2,000 miles, along with a masturbating dog and a coffee can of Tremblay’s father’s ashes; as expected, chaos ensues.

But we’re not talking about making a wrong turn as in “Dumb and Dumber,” or suffering from the bad jokes of an obnoxious companion courtesy of “Planes, Trains & Automobiles.” “Due Date” takes it to the next level, putting the characters through some of the most ridiculously dangerous and illegal escapades imaginable.

And yet, after the cop cars are flipped over and Peter is busted out of a Mexican jail for drug smuggling, the characters face zero consequences. It seems ridiculous to qualify the events in “The Hangover” as plausible, but compared to “Due Date,” ripping out your own tooth seems pretty tame.

Downey Jr. and Galifianakis have their funny moments, but ultimately, they are the same characters they’ve portrayed in movies past. Downey Jr. is best known for his dry sarcasm and underlying charm, yet Peter lacks any likeability, coming off as a total prick. He’s Tony Stark with all the snark, but none of the redeeming qualities that make Downey Jr. such an appealing actor.

And while Peter mocks Ethan relentlessly throughout the film for his stupidity and social flaws, his cruelty seems almost justifiable based on how horrendously awkward and gross Ethan’s character is. There’s only so much onscreen time Galifianakis can claim without making an audience uncomfortable, and after a half-hour of absurdity, Peter’s not the only one who wants to strangle Ethan with his own scarf. It’s strange watching Galifianakis as a leading role, trying to hold his own against Downey Jr., and even stranger when the audience is meant to feel for Ethan’s character as the film progresses.

While their friendship “blossoms” and Ethan is presented to the audience as more than a gimmick, we’re expected to find him loveable, rather than repulsive. Unfortunately this just doesn’t translate. As Galifianakis’s character in “The Hangover” is fondly remembered as random and absurd, he was loveable in the sense of his loneliness.

Who could forget his impromptu performance of “We’re the three best friends that anyone could have”? How could you not feel for that guy? Unfortunately, Ethan Tremblay’s ignorance of social norms does not translate into any kind of sympathy, creating a cringe-worthy performance that would have made walking the 2,000 miles a more viable option.

Ultimately, “Due Date” came with high expectations and little rewards – although there was a handful of scenes that had the audience laughing, despite how preposterous the context might have been. As a director, Phillips is a master at mixing music into an effortlessly cool cinematic experience, making any guy in their thirties look like a total player as he walks in slow motion to a killer soundtrack. And yes, after seeing this film you will leave the theater with some awesome one-liners and fond memories of Downey Jr.’s sexy ruggedness.

However, “Due Date” is no “Hangover,” and after sitting through 100 minutes of the road trip from hell, a 24-hour blackout in Vegas doesn’t look so bad in retrospect. How many days until “Hangover 2”? Start the countdown, and sit this one out.

2 stars out of 5