She’s just been fired from her $9-an-hour cashier’s job at a dollar store; she’s late on the rent to her tolerant if unpredictable landlady, the aptly named Dottie (Rosemary Knower), and worn out from caring for the unseen Joyce, her grown-up, mentally disabled daughter. If that is not a recipe for audience sympathy, what is?

Well, not so fast. Or at least, not so simple. How can it be that everyone in “Good People,” even Margie, clearly recognizes the unhelpful or hurtful inclinations of others, but can’t see it in themselves? “You have to be a selfish [expletive] to get anywhere,” one of the characters, who hasn’t gotten anywhere, opines. The notion of selfishness — the extremes we go to, looking out for No. 1 — materializes in the play again and again: in the ease, for example, with which Dottie gravitates to the idea of throwing Margie and her daughter out; in the craven encouragements of Margie’s pal Jean (a hilarious Amy McWilliams, as the play’s throaty voice of plain talk); in the tentative yet heartless way that Stevie (Michael Glenn) dismisses Margie from her job.

And most combustibly, in the wonderful culminating scene in Act 2, when Margie sandbags her old flame Mike and his young wife Kate (the outstanding Andrew Long and Francesca Choy-Kee), in their luxurious home in a Boston suburb, where Margie is so out of place that Kate mistakes her for a delivery person. (Kate’s ethnicity is further fuel for the fire.) Put off by Mike’s skittishness at offering her any kind of help, Margie lashes out with a story calculated to drive a wedge between the doctor and his spouse — an act that sparks a passionate rebuke from Choy-Kee’s Kate, expertly spitting the bile back in one of the evening’s best speeches.

Maxwell, artistic director of Ontario’s Shaw Festival, has a surefire ear for comedy: This “Good People” is even funnier than the excellent 2011 version with Frances McDormand at Manhattan Theatre Club. The amusing results are thanks in big part to the convincing mix of abrasiveness and camaraderie conveyed in the kitchen and in Bingo scenes among Day, McWilliams and Knower, and the success of endearingly risible bits, such as Dottie’s habit of gluing hideous Styrofoam balls to ceramics and selling them as curios.

Day, who last appeared at Arena as a New Age-y Californian in 2009’s “The Quality Life” and before that was Lizzie Curry in a revival of “The Rainmaker,” has in Margie a role that seems ideally contoured to her talents. There’s so much conviction in her weary though undefeated countenance that you can’t help but root for her; when she shows up at Mike’s Boston office, offering herself as a janitor, the courage tinged with vulnerability makes her irresistible. To Mike’s declaration that his circumstances are comfortable, Day ensures that Margie’s reply — “I guess I’m uncomfortable” — strikes just the right wistfully ironic note.