Watching Robert Downey Jr and Zach Galifianakis make their odd-couple way across America by car it is impossible not to recall Steve Martin and John Candy's similar journey in Planes, Trains & Automobiles. The chief ingredients are the same. A man desperate to get home finds he is unable to extricate himself from the company of an oddball slob but, along the way, discovers that despite outward appearances, deep, deep down his companion is lovely on the inside.

　　 Here, Downey's Peter is less a stuck up snob than a workaholic with anger management issues, while Galifianakis' Ethan is a wannabe actor with a hippy vibe. And this time around it is pregnancy that makes the trip imperative - with Peter's wife due to give birth by Caesarean on a specific day of the week. The funny thing about ingredients, though, is that even if they are similar, the recipe doesn't always turn out the same - and the one thing missing here, that John Hughes' film had in bucketloads, is heart. Martin and Candy's physical and spirtual journey was notable for its soft, feel-good centre. The odyssey of Downey and Galifianakis, meanwhile, is composed of far too many sharp edges and far too little warmth.

　　 If it isn't as emotionally satisfying as its forebear, however, this film - from the team behind the incredibly successful The Hangover - is still entertaining to a point. The humour springs not from the characters - whose arcs are disappointingly weak - but from the（英文影评） situational comedy they find themselves in. Among the scrapes along the journey are a wrong turn that takes them, stoned - it's a long and not-as-humorous-as-it-should-be story - to the Mexican border and an incident which sees Ethan's father's ashes (stored in a coffee can) engaging in some minor peril with a coffee machine.

　　 But for every mildly funny episode, there seems to be a matching moment of crudity without the comedy. Spitting is played for laughs on more than one occasion and one punchline involves a small child being punched in the stomach, while a scene involving Ethan and his dog masturbating pushes things too far for general comfort and yet not far enough to take it into the realms of proper 'gross out'.

　　 What saves the film are the performances. Downey is always watchable and brings a surprising amount of sympathy to Peter, considering that his character is virtually the same at the end of the film as he is at the beginning. Galafianakis, too, manages to imbue his hapless actor with a level of charm that goes beyond the script, although he is always in Downey's shadow. Predictable as the urge for a box of popcorn as you enter the cinema but just about diverting enough. Those really looking for laughs, however, would be better off dusting down their copy of Planes, Trains & Automobiles and watching that instead.