Happy-Go-Lucky (15)

A feelgood Mike Leigh movie? I never thought I'd see the day, but Happy-Go-Lucky, as its title indicates, really does work on the sunny side of the street. It's hard to know from the famously intense collaboration Leigh conducts with his actors exactly what tone each film will take, but somewhere along the rehearsal process on this one we can assume the words "optimism", "warmth" and "happiness" were bandied around.

True to form, however, there are challenges in it which some will find as difficult to meet as those found in the darkest and grimmest corners of Leigh's dramatic catalogue.

Sally Hawkins, carrying the movie, plays the 30-year-old Poppy, a London primary-school teacher who embraces life with a smile or a shrug. This is a woman who can cycle through the capital's streets – a test of mental and physical endurance in itself – and maintain an innocently cheerful expression. Even when she finds that her bike has been stolen, there's no foot-stamping: "Didn't even get a chance to say goodbye", is all she says, as if it were a dear old friend.

The heroine of Leigh's previous movie, Vera Drake, was a similarly good-hearted, gregarious sort, but then the plot was lying in wait to destroy her. Poppy also confronts the dark side of human nature, but now the stakes aren't so high and she, for all her bright-eyed chirpiness, is nobody's fool.

Watch the Happy Go Lucky trailer

Pollyanna that she is, Poppy turns the loss of her bike into an opportunity – she'd always meant to learn to drive, and so she books lessons with Scott (Eddie Marsan), a man whose social unease is initially quite funny. He is exasperated by Poppy's cavalier driving and verbal teasing ("Bear with me" – "Is there? I can't see him!") and only by degrees is he revealed to be bitterly paranoid and aggressive. In the meantime we sit back and watch Poppy go about her life, boozing with flatmate Zoe (Alexis Zegerman), visiting a physio, joining a flamenco class, and smiling all the way.

The film rather meanders through its first hour, and caused me to feel a twinge of guilt for not quite warming to Poppy as I ought. Intense cheeriness, like intense misery, can become oppressive.

Leigh, perhaps sensing an absence of friction, takes a bizarre little detour halfway through when Poppy, walking alone at night, strays into the company of a homeless man and listens, sympathetically, to his incoherent patter. The man disappears, for good, and the scene has absolutely no follow-through. Is this Leigh's way of reminding us of the madness and despair just below the city surface?

Not much more successful is Poppy's visit to her younger married sister Helen (Caroline Martin), smugly pregnant and sniping at Poppy's carefree life. Leigh hates lower-middle-class aspiration almost more than anything else – the barbecue, the patio, the talk of "the property ladder". He seems almost to admire Poppy because she still lives in scruffy bedsit-land. Would he like her as much if she earned better money and bought a place of her own?

At times the optimism feels a little too willed. When Poppy notices that one of her pupils is bullying others, she alerts a colleague, who in turn gets a social worker on the case. However, instead of pursuing this story, the film effects a romantic attraction between Poppy and the social worker.

Every time the mood looks threatening, Leigh make a determined sideways swerve. It's not that Leigh makes Poppy an unrealistic character; he just makes her too straightforward. How much more interesting if her relentless buoyancy were something hard-won, a compensation or a cover for some private heartache.

Sally Hawkins, a wonderful actress, plays the part almost entirely on one note, and it sounds like the little squeak of laughter with which she caps every sentence. The performance is an honest attempt to make goodness credible, but it needs a few edges to it. There are moments when her exuberance prompts a response that seems genuinely off-the-cuff, usually when Poppy has to defuse the tension while next to her driving instructor ("Are you a satanist, Scott?").

My favourite moment of all comes when she is introduced to the fierce flamenco teacher and, hoping to impress, performs an awkward little curtsy; the moment is so touching and funny that it made me want to like her in spite of the exhausting displays of good cheer. That, I think, is brilliant acting.

Happy-Go-Lucky is Mike Leigh's sunniest film, though some way short of his best. Aside from the driving instructor's waxing fury, it lacks strong dramatic propulsion, especially in its first half. Perhaps that is the director's point, that life simply bowls along in its largely eventless way, sometimes funny and charming, more often not.

But film – art – has an advantage over life in being able to select and discriminate; that's how it takes on shape and meaning. There's not much evidence of either here.