Jeff, Who Lives at Home (15)

I love titles with a comma in them. Whether it's enigmatic, like Craig Raine's book of poems The Onion, Memory, or plaintive (Will You Please Be Quiet, Please?) or exhortatory (Rabbit, Run) or elegiac (Larkin's Going, Going), the comma makes a little hook to lure you in. With films, it crops up in the good (Happy, Texas), the bad (Dude, Where's My Car?) and the indifferent (Lust, Caution), but the pause it creates is cheering in a high-speed digital age where punctuation is losing its potency.

The brothers Mark and Jay Duplass have hit the bull's-eye with their troubled family comedy Jeff, Who Lives at Home. The comma here speaks of hesitation, of doubt. Jeff (ahem) is a bit sad, a 30-year-old deadbeat who lolls about in his mother's poky basement, puffing on his bong and watching TV. His opening monologue, spoken for some reason into a tape-recorder, testifies to his obsession with the M. Night Shyamalan alien-visitation movie Signs. This in itself is a sign – that Jeff's pretty gullible. That comma also implies a kind of stasis. Jeff has been living at home for far too long; he needs to get out more. Or at least watch some better movies. As played by Jason Segel, he's a pudgy man-child who doesn't know how to help himself – rather like Jonah Hill in the previous Duplass movie, Cyrus, who also lived at home.

You can even hear the comma in the way Jeff's frazzled mum (Susan Sarandon) talks to him on the phone. All she wants is for Jeff (sigh) to do an errand for her, namely go and buy some wood glue to fix the broken kitchen shutter. Just that one thing. It so happens that Jeff has had another phone call that morning, an agitated voice he doesn't recognise asking him if "Kevin" is there. Who's Kevin? Jeff hasn't a clue, but when he goes out on his glue-buying mission he's primed for any sign of Kevin, believing it to hold the key to his destiny. That's what you get for obsessing over M. Night Shyamalan. Wandering through the unexceptional suburbs of Baton Rouge, he bumps into his older brother Pat (Ed Helms), who's not much more clued-up than Jeff. He's just bought a Porsche he can't afford, and his outraged wife, Linda (Judy Greer), has just tipped her breakfast over it. Pat's an angry, unfulfilled man, but he considers himself superior to Jeff insofar as he's got a job (paint salesman) and takes "business meetings" (in Hooters, unfortunately).

What follows is a day of escalating misadventures. Jeff and Pat don't get along, but they're forced into an uneasy alliance when Pat spots Judy getting into a car with another man. They pursue her around town, though their investigation will reveal more about Pat's inadequacy than it does about Judy's possible infidelity. Meanwhile, their widowed mother, Sharon, is fretting through the day in her dreary office job, her time spent in a cubicle doing something on the phone. Her mouth a rictus of middle-aged disappointment, she's at first exasperated by the mysterious flirty emails in her inbox, then intrigued, and then rather excited when she realises that her secret admirer is probably – be still, my heart! – a co-worker. Sarandon judges Sharon's brittle mixture of hopefulness and dismay just right: you know this woman, and you know what it means to her when the prospect of last-gasp love bobs on to the horizon.

Actually, all the main performances here are superb. Ed Helms, from The Hangover, has played worrywarts and oddballs and deluded jerks before, but this is something else. Sporting the goatee of a would-be player, he looks even further out of his depth than usual. The remarkable thing that the script allows him to do is claw back some humanity from a terribly unpromising start. As his defeated wife, Greer works small wonders, her pleasant birdlike features struggling to express the hurt she's had to endure in keeping their marriage alive (she played a somewhat similar role in The Descendants). Segel holds it all together as Jeff, whose dreamy vagueness is charming to us but probably quite maddening if you had to live with it. Segel's comic trademark is a sort of teddy-bear tenderness that's just the right side of ingratiating: he wants to please (he's desperate to please) but knows his awkward body shape and diffident manner put him at a disadvantage. He played against this type in I Love You, Man – another comedy with a comma – as a free-spirited lothario to Paul Rudd's uptight everyman. It didn't quite work, but there's something about Segel that puts you on his side; it might just be his amiable modesty.

With all this comic gas in the tank, it's puzzling that the film isn't completely wonderful. It may be to do with its somewhat disjointed nature. The Duplass brothers are often discussed within the critical bracketry of Mumblecore, a brand of low-key film-making that anchors life in a shrugging, humdrum spirit of realism. The nervy camerawork complements this mood, zooming in close on characters as if to say "you've got be kidding". Yet the plot depends on the sort of outlandish coincidences that only a fable – or possibly a David Lynch dream movie – could get away with. I lost count of the number of times these people kept bumping into one another; by the time of its breathless finale, you're wondering if the Duplasses are secretly taking the mick. Jeff, believing that the universe has a plan for him, keeps doggedly pursuing the signs. Of course we want things to work out for him, and for his family, but only if the film stays faithful to the sort of people they are. And this sort don't live fantasy lives – they live in wistfulness and uncertainty, with commas all over the place.