Director Mike Leigh shows how sweet it is in Happy-Go-Lucky

Poppy, the high-spirited heroine of the comedy Happy-Go-Lucky, wants to cheer people up. Sally Hawkins plays Poppy as such a bubbly, joking motormouth, she reminded me of a remark by C.S. Lewis: "She's the sort of woman who lives for others – you can always tell the others by their hunted expression."

Poppy all but insists on connecting with friends and strangers in the name of making them happy. In the film's first scene, she doggedly kids around with a sullen bookstore clerk who'd rather keep his silence. For such a warm, well-meaning person, Poppy can generate some tension wherever she goes. It's as though all of London has a hangover except her, and she's wandering around beaming, "Smile!"

Happy-Go-Lucky follows Poppy for a few weeks in her life as a single, 30-year-old schoolteacher. Writer/director Mike Leigh reveals her to be more than a Pollyanna or a grinning idiot. Poppy has substance that surpasses nonstop jokes, candy-colored outfits and the occasional chicken cutlets in her bra. Leigh calls Happy-Go-Lucky an "anti-miserablist film," signaling a change of pace from his often grim portrayals of working-class struggle. The film suggests that happiness isn't just a state of mind, but a choice that some people prove simply unwilling or unable to make.

A schoolteacher with unerring instincts for educational play, Poppy learns different facets of maturity from other teachers such as her flatmate Zoe (Alexis Zegerman), who frequently serves as Poppy's sardonic straight man. Poppy takes flamenco lessons from a fiery dance instructor (Karina Fernandez) whose passions prove so explosive, she rants and breaks down before one class. Poppy seems to learn pride and courage from her, and in the film's enigmatic centerpiece, Poppy makes a warm but nonverbal connection with an incoherent homeless man (Stanley Townsend).

Happy-Go-Lucky's most charged scenes involve Scott (Eddie Marsan), a terrier-like driving instructor who's so tightly wound and implosive, he can scarcely relate to other people. Poppy's irreverence winds him up, but he refuses to end the lessons, declaring, "I never give up on a pupil." Scott's dedication inspires Poppy's decision to draw out a young bully at her school. But can the better angles of Poppy's nature get through to Scott, whom Marsan plays as a nerve-wracking knot of bigotries and frustrations?

In Leigh's signature method for developing his films, the director spent months in workshops with Hawkins and the other actors to develop their roles. He held one-on-one sessions followed by group improvisation exercises in which the actors knew only what their characters would. The results, in films such as Secrets & Lies and Life Is Sweet, frequently feature performances with unusual depth and realism, and stories that defy conventional film scripts' predictable beats. The events in Leigh's films seem less predetermined, guided by the lives of his characters more than the dictates of a genre.

Happy-Go-Lucky offers a telling (but probably unintentional) contrast with Hollywood's spate of man-child comedies, particularly such terrific films as The 40 Year-Old Virgin. The protagonists of such films usually prove to be guys who amount to oversized kids and eventually put away their childishness over the course of the film. In Hollywood movies, women of Poppy's age invariably obsess over their ticking biological clock as well.

Refreshingly nonjudgmental, Leigh ultimately asserts that Poppy doesn't need to grow up – she is grown up. She may crack rapid-fire quips like Ricky Gervais on "The Office," but Hawkins reveals Poppy to be a young woman of unexpected perceptiveness and sensitivity. She's always "on," but that doesn't mean she isn't paying attention. After nearly two hours with Poppy, Happy-Go-Lucky leaves us a little exhausted and exasperated with her, but also convinced that if you need a role model, you could do a lot worse.