Cedar Rapids (15)

Starring: Ed Helms, John C Reilly, Sigourney Weaver, Anne Heche

Comedy seldom comes as good-natured as Cedar Rapids, a fish-out-of-water story in which a Midwestern innocent discovers sex, drugs and cream sherry in the big city.

The twist is that the innocent in question is approximately 35-years old. He would be Tim Lippe (Ed Helms), pronounced "lippy", a straight-arrow insurance agent in Brown Valley, Wisconsin, and perhaps a distant relation of Steve Carell's Andy Stitzer in The 40-Year-Old Virgin. True, Tim has recently got it together with his old grade-school teacher Macy (Sigourney Weaver), but rather in the spirit that a Boy Scout might do odd jobs for his auntie. He's essentially a bachelor boy, with the emphasis on the boy.

But lo, Tim's world is about to change. A lowly employee at the Brown Star Insurance Company, he finds himself a last-minute substitute for the annual convention of insurance agents at Cedar Rapids, Iowa, which in this context counts as Sin City. His mission is to bring back the coveted Two Diamonds trophy, awarded for professional integrity and won for the last three years by Brown Star. His boss packs him off with a warning to steer clear of a notorious hellraiser by the name of Dean Ziegler. Tim, arriving at Cedar Rapids with a suitcaseful of sensible knitwear and pious intent, can hardly believe the luxury of the convention hotel, with its credit-card passkey and swimming pool that pongs of chlorine: "It's like I've come to Barbados or somewhere," he coos. (We are asked to believe that Tim has never stayed in a hotel before). As for his very drab-looking junior suite: "Check out these digs!"

They are digs he happens to be sharing with two fellow insurance salesmen. One of them is mild-mannered Ronald Wilkes (Isiah Whitlock Jr), whose idea of fun is settling down to watch "the HBO programme The Wire"; the other (uh-oh) is the aforementioned Dean Ziegler (John C Reilly), a potty-mouthed blowhard who treats everyone with the same bullying camaraderie. Soon Tim's at the bar knocking back shots with them and Joan, aka Foxy (Anne Heche), another convention regular who's got a tiny crush on the wide-eyed newcomer. She also responds to Tim's honest reverence for the job – it was an insurance agent, he explains, who saved him and his mum from penury after his dad died. As Tim delivers this speech, both of them seated on swings in a kid's playground, you keep waiting for a comic spike to burst the balloon of sincerity – but it doesn't come. This is not a place for the cynical laugh.

That's a surprise, given director Miguel Arteta's track record with comedy (Chuck & Buck, The Good Girl), while Phil Johnston's patchy script goes for mild rather than wild. What the film has going for it is the presence of Ed Helms, one of the funniest actors on screen at the moment. A graduate of Saturday Night Live – like almost every American comedian you've ever heard of – Helms first came to general notice as one of the trio enduring The Hangover. But he's really shone as Andy Bernard, the a cappella-obsessed cuckold of The Office (US version). Helms uses his terrific singing voice to sudden and hilarious effect – his piano-led ditty "What Do Tigers Dream Of..." in The Hangover is a classic – and I looked forward to the moment he would get to show it off here. Sadly, it's a karaoke scene in which he sings a lame-but-sweet tribute to his insurance company back home: his audience hardly know how to respond, and you may feel the same way.

You can't really resist Helms for long, though, even when he's handed material as threadbare as this. With his slightly too large clothes and his neat preppy haircut, he embodies an America at odds with itself, outwardly confident in manner but with an incipient panic behind the eyes. Tim discovers in the course of his weekend that some of the people he most revered are charlatans and hypocrites, and yet he never sheds his delightful aura of ingenuous goodwill. He still believes in the possibility of integrity, still behaves like a gentleman, and still uses his favourite expression of approval, "super-awesome". He makes the learning curve of this square something charming, even if his cluelessness doesn't always ring true. I didn't swallow, for instance, the idea that he's never been on a plane before, or booked into a hotel; on the other hand, I could quite imagine his being solicited by a prostitute, who asks him for a cigarette. No, he doesn't have a cigarette for her – and by the way smoking will send her insurance policy through the roof – but he can offer her a butterscotch toffee.

Cedar Rapids also scores points on its ensemble playing, from Reilly's loutish woo-hooing to Isiah Whitlock's deadpan drollery and Anne Heche's surprisingly tender portrayal of a woman who seems to live for these occupational jollities (there's more sadness than guilt in the phonecall she makes to her husband from another man's bedroom). It has the effect of warming one's heart towards Midwestern folk, if not to the extent of yearning for a weekend with them. But it's an odd comedy that provokes admiration more for its acting than for its gags. I kept wishing it could be, well, not super-awesome, exactly – just better.