Todd Phillips and Zach Galifianakis, the director and break-out star of last summer’s box office behemoth, The Hangover, go for gold again (with the added presence of man of the moment Robert Downey Jr.) in new comedy Due Date.

Downey Jr. is successful, yet highly-strung architect Peter Highman, who is preparing to travel home to Los Angeles from a business trip in Atlanta for the imminent birth of his first child. Enter aspiring actor Ethan Tremblay, who after a previous altercation at the airport’s taxi rank, boards the same flight of Highman and manages to get the two of them (in a rather contrived gag) thrown off the plane before take off.

Grounded and now unable to book another ticket due to the incident, Highman (who discovers his wallet and personal belongings are still on the plane he was ejected from) is forced to take Tremblay’s kind offer of a lift, cross-country, in a hire car. The two set off (with Tremblay’s pet pug in tow) and Highman soon discovers to his horror that he’s accepted the ride from hell, which will not only test his mental well-being, but will also push him to the physical limits too.

The odd couple and road movie scenario has been successfully combined in the past (Midnight Run and Planes, Trains and Automobiles are two glowing examples) and there’s much comic material to be mined here too. Director Phillips also knows not to mess too much with the winning formula he created for his previous feature, which Due Date shares a couple of similarities to, other than the fact that they both feature Galifianakis as a slightly demented and off-kilter human being.

The race against the clock device is present again in the narrative here, although there is more of a leisurely pace this time, as Downey Jr. has a few days till his wife is due to give birth – much longer than the few desperate hours the boys from The Hangover had to contend with. That difference really works to this film’s advantage, as there is space for character development and time to breathe between scenes. Philips also manages to capture some really beautiful cross-country scenery throughout too.

Both films also hinge on simple premises, and it’s to the credit of Downey Jr. and Galifianakis here that they bring so much to the various comedic predicaments and awkward situations they are constantly involved in. Some of the gags themselves however, feel a little old and worn at times. The horny dog routine has been seen hundreds of times before and you can pre-empt where the laughs will occur in the plotline involving Tremblay’s father ashes (which he keeps in an empty coffee tin) as soon as he reveals them, although the two actors manage to bring something new to those scenes which (mostly) help dismiss an annoying sense of deja vu.

Galifianakis is a real joy here playing the slightly effeminate, wannabe Hollywood star who perms his hair and smokes a gargantuan amount of weed, all the while claiming it’s for a medical condition. Just his presence in a scene is enough to elicit big laughs – he doesn’t even need to open his mouth. The two actors have fantastic chemistry together, with Galifianakis’s childlike sense of the world perfectly clashing with and complementing Downey Jr., and all his pent-up fury and repressed aggression.

In one hilarious scene, Highman (finally living up to his name) is completely wasted from passive smoking and fails to notice that Trembly, in his search for petrol, has driven into a queue for the entrance to Mexico, having confused it for Texaco. The border arrest of one of the duo and the ensuing escape sequence is a entertaining mix of action-comedy cinema.

The duo are more than ably supported by a colourful cast of characters too, comprising of the likes of Danny McBride, Jamie Foxx (Downey Jr’s recent sparring partner in The Soloist) and Juliette Lewis, whose annoying son finds himself on the receiving end of Downey Jr’s anger in one of the film’s funniest, yet shocking scenes.

Ultimately, Due Date doesn’t add anything new to the comedy genre or attempt to inject new life into the cinematic road trip conventions we’ve come to expect, but it’s a fun and often hilarious 90-plus minutes, helped largely by two Hollywood performances at the top of their game.