“I hope people ask me, ‘Where did you find that local actress?,’” Ben Affleck told Amy Ryan when he cast her as a wreck of a single mother in his directing debut, Gone Baby Gone. When Ryan showed up on the Boston set in ratty hair, muddy makeup, and a getup that wouldn’t disgrace a smalltime hooker, a zealous production assistant refused her entry. The actress stood quietly with the Dorchester locals Affleck had hired for the shoot until a producer rescued her. “You have no idea the vote of confidence you just gave me,” she told the mortified assistant.

Gone Baby Gone is Ryan’s overdue big break, but if you’re a regular moviegoer, you’ve seen her around a lot, probably without knowing it. Best known for her recurring role as Officer “Beadie” Russell on HBO’s The Wire, Ryan has one of those lived-in, ordinary faces that often strand talented young actresses in bit-part hell. In person she’s feline-pretty, with a low, melodic voice and a quick wit, but that hasn’t stopped her from getting cast as the reliable wife. She was Chris Cooper’s star-struck spouse in Capote, and if you blink you’ll miss her hoisting babies onto her hip as Steve Carell’s benign sister-in-law in the new romantic comedy Dan in Real Life.

Like it or not, though, her specialty has become the febrile single mother. She is extremely good for all 10 of her movie minutes as Ethan Hawke’s bitter estranged wife in Sidney Lumet’s Before the Devil Knows You’re Dead. Ryan first caught my attention in Lodge Kerrigan’s Keane, in which she puts enough quiet dignity into her role as a down-on-her-luck single mom who leaves her child in the care of a schizophrenic to draw our sympathy.

Gone Baby Gone requires a similar stunt of Ryan’s character Helene McCready, the wild, profane and frankly slutty mother of a little girl who’s disappeared, but Affleck had not see Keane when he cast the actress. “I think they cast me because I have bad, stringy hair,” Ryan tells me over breakfast at the Four Seasons, where her co-star Michelle Monaghan briefly interrupts by creeping up behind her chair to say boo. “And weak lachrymal glands. I cry really easily — my friends call me Cryin’ Ryan.”

Whatever: Ryan plays Helene with a shrewd sense of comedy that bravely complicates that self-righteous term, “in the best interests of the child,” electrifies what would otherwise be an intelligent bummer of a movie, and evokes — if not sympathy, then at least a grudging respect for this hopelessly ill-equipped, yet loving mom. “We have a child at risk,” says Ryan, “and a mother who seems not to care. She’s doing her best, but unfortunately her best is terrible.”

Much of Helene’s character was right there in the script, which Affleck adapted with Aaron Stockard from a novel by the gifted writer Dennis Lehane. “Unlike a lot of two-dimensional versions of that character I had played, all of it was very much on the page,” says Ryan. “I realized these goal-posts are very wide, and Ben pushed them even further. The safety for me was that he knew this Dorchester world, and he was not going to make fun of it. But he told me that if my part failed in the movie, we were screwed.”

Nothing could be more different from Helene’s hardscrabble life than Ryan’s own secure Queens upbringing as one of three sisters raised by supportive parents who took their daughters to the theater in Manhattan as often as they could, mostly to musicals. It wasn’t until she enrolled at New York’s High School For the Performing Arts, says Ryan, “that I learned you could be in theater without singing.” Ryan did well enough there to earn two Tony nominations, one for her Sonya in Uncle Vanya, the other as Stella opposite Patricia Clarkson in A Streetcar Named Desire. Steady television work has paid the bills, and roles in low-budget indies — what she calls “changing clothes in gas stations” — have modestly fattened her resume. (She plays Neal Cassady’s grass-widow wife Carolyn in Noah Buschel’s Beat biopic Neal Cassady, which is currently making the festival rounds.)

Next year she gets to haul out those hooker threads again for a role in Clint Eastwood’s The Changeling. So the foreseeable future, at last, looks good for Ryan, whose wish list includes directors as diverse as Paul Thomas Anderson and Mike Leigh. Whatever happens, though, she still holds tight to the advice of Clarkson, a close friend and an inspiration along with Catherine Keener and Laura Linney (“solid, smart, sexy women who never bring their ego to the movie”). “If you keep making money choices it’s gonna bite you in the ass,” Clarkson warned her. Still, Ryan has advice of her own for fledgling actors: “Start an IRA.”

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