What could possibly be unfunny about Zach Galifianakis and Robert Downey Jr. stuck in a car together on a cross-country trip? This concept, used as a primary selling point for director Todd Phillips' sophomore effort, "Due Date," uses the same rubric, and some of the same jokes, as his directorial debut, "The Hangover" (2009).

That film was also marketed as a fail-proof film concept: four funny and recognizable faces collectively black out in Las Vegas and go through what many people know from personal experience—piecing together the events of the previous night. Both films follow men on wild and absurd odysseys while their female counterparts wait for them patiently to return.

The problem of this film-making style—based around their basic concept and selling points rather than the desire to make a developed, quality film—is that Phillips has no real drive to create any kind of character development since the attraction of the film seems to come from nothing more than the presence of multiple funny personalities on the same screen at the same time.

Since both Downey and Galifianakis and their situation are funny, the film can't fail, right? Wrong. "Due Date," which casts Downey as Peter, a high strung architect in need of a ride, and Galifianakis as Galifianakis (in the form of Ethan, an aspiring actor), puts all of its proverbial eggs in the basket of good chemistry between the two characters. When that chemistry fails time and time again, so does the film.

Phillips is not striving for originality in the plot of the film, which is essentially a "Planes, Trains, and Automobiles" remake, as well as being too similar to the more-entertaining and better-acted "Get Him to the Greek" of the summer.

Though "The Hangover" was admittedly amusing at times, "Due Date" banks too hard on the hope that audiences will remember the former and want more, and hopes that the recycled jokes, which are many, will seem fresh.

Galifianakis plays the exact same character, except that he wears a thespian hat, walks with a slightly more dramatic gait, and his baby from "The Hangover" has been replaced with a pet bulldog.

Peter is cold and unsympathetic, and though he is supposed to have his volatile anger issues resolved by the end of the film, the transformation is not convincing at all. He has several temper tantrums at Ethan that are spread over the course of the film, and we have no reason to believe that this trend has come to an end by the film's finale.

Not that we can blame Peter for lashing out at Ethan; Galifianakis' often-annoying shtick, permissible in "The Hangover" where it was diluted with the routines of his three co-stars, becomes tedious and unbearable here, where he gets significantly more solo screen time.

However, we presumably are supposed to hope for a kind of meaningful blossoming friendship between the two men that are literally on screen (one or the other, if not both at the same time) for the entirety of the film. But after being pestered by Ethan's antics and off-put by Peter's emptiness as a character, this reconciliation is hard to root for.

The tagline of "Due Date" is "Go outside your comfort zone." Ultimately, if Phillips is going to have any kind of staying power as a writer or director, he must venture outside his own comfort zone to try and make films that rely less on doggedly persistent attempts at shock-value laughs.

Rating: 2/5 stars