Hollywood is finally chasing Amy

Amy Ryan knows all about being brought back down to earth. Nominated for an Academy Award this year for her breakout performance in Ben Affleck's credible directorial debut Gone Baby Gone, she went home empty handed. And then? "The day after the Oscars, I was back in New York and dropping off my laundry at the laundromat."

After losing out in the Best Supporting Actress category to Tilda Swinton for Michael Clayton, having to see to your own dirty knickers is hardly compensation. But Ryan admits it was the perfect antidote to the months of pre-Oscar hype. "It's all about checks and balances," she says.

A month after the Oscars, we meet in London. Ryan has just about recovered from the campaigning for that little golden statue. "It's like training for the Olympics," she says. "You're exhausted, you can barely breathe, you haven't slept. By the time the day arrives, it's like, 'Thank God it's here!'"

She was not the only dark horse this year – coming between the 83-year-old Ruby Dee for American Gangster and 14-year-old Saoirse Ronan for Atonement – but arguably she had the most to lose. With a workaday career stretching back almost two decades, Ryan, 38, knew it was now or never.

The omens were good. When Gone Baby Gone was released in the US last October, it arrived in the same month as two other films featuring Ryan, the Steve Carell comedy Dan in Real Life and the smart Sidney Lumet thriller Before the Devil Knows You're Dead, in which she played Ethan Hawke's ex-wife. "There were a lot of people saying, 'I knew it! I knew you'd have this time!'" she says. But, just as she refused to buy into the Oscar hype, Ryan stayed grounded. "As lovely a movie as Dan in Real Life is, there was no real part there for me [she was Carell's sister-in-law]." This was all too familiar: too often she'd gone unnoticed in supporting roles, from Kenneth Lonergan's You Can Count On Me to Lodge Kerrigan's Keane.

This could have happened with Gone Baby Gone, too. Ryan is almost unrecognisable in the film, playing a drug-addicted single mother in Boston. In person, she's striking – green eyes, honey-coloured, flowing hair, cheekbones to die for – but as Helene McCready she's something else. Her skin has a deathly pallor, her hair is lank and her bubblegum-pink nail polish (picked out by Affleck) is chipped and worn. So convincing is she that when she left her trailer on the first day of shooting in full costume, the police would not let her through the barriers keeping the public at bay.

Based on the novel by Dennis Lehane, Gone Baby Gone, a gritty neo-noir thriller, gained considerable notoriety in Britain after it was pulled from the London Film Festival last October and then shunted from its December release date to this June. Rightly, the film's distributor, Buena Vista International UK, was sensitive to the uncanny similarities that the plot, which begins with the disappearance of Helene's young daughter Amanda, bears to the Madeleine McCann case. So concerned is the company that I was on strict instructions not to ask Ryan about the real-life case.

As some critics have pointed out, the story – which follows two private detectives (Affleck's younger brother Casey and Michelle Monaghan) attempting to track Amanda down – has more in common with the Shannon Matthews saga. Still, what cannot be denied is the power of Ryan's performance, overshadowing veteran co-stars Morgan Freeman and Ed Harris, who play cops.

Ryan compares working with Harris to the time she went skydiving. "I was terrified as the plane was going up, but as I was standing at the exit door, I remember thinking, 'Well, you're going to die or have the time of your life. So pick one.' And it's kind of like that, working with Ed Harris."

Ryan certainly made a leap into the unknown. Her frightening, twisted performance represents "an all-too-familiar character in America", as she puts it. "She's a single parent who doesn't have the luxury of education or healthcare or childcare or any form of therapy, so turns to self-medication, drinks too much and takes too many drugs. It's a woman caught in an endless cycle. When I look at this movie and think of the young girl we have so much compassion for, I keep in mind there was a time when Helene was that four-year-old girl, raised in a similar situation. The question is, 'How do you stop this cycle and take care of our children so they don't grow into monstrous adults? How do you break that cycle?' And that's who she is."

Raised in Queens, the youngest of three girls, Ryan may not know the seedy world Helene inhabits, but her upbringing was hardly fancy. Her mother was a nurse and her father owned a small trucking company, but it was they who introduced Ryan and her sisters to the theatre and movies. While her mother "has musical talents" and one sister wrote poetry, it was Ryan who saw the arts as a career. "I feel like, in some ways, everyone else in my family was far more creative than I ever was or am," she says. "But somehow, I'm the one who went ahead with it. For them, it was a private matter."

When she was 14, she went to study theatre at New York's public High School for Performing Arts – "which is basically Fame!" she chuckles. Graduating, she played in a touring Biloxi Blues. And theatre remains her life-blood; in 2000, she was nominated for a Tony for her role in an Uncle Vanya, and five years later, her Stella Kowalski in A Streetcar Named Desire saw her gain a second nomination.

Ryan worked steadily in television – including a role as Officer "Beadie" Russell in the feted HBO show The Wire – but film remained elusive. She says always tried to be hopeful rather than hateful regarding rejections: "I think getting bitter is the fastest way out of this business."

Even when she did score a big film, it was always in a role too minor to get noticed – such as her part as a neighbour holding a toddler in Steven Spielberg's War of the Worlds. It was her work as the star-struck spouse to Chris Cooper's lawman in 2005's Capote that changed things. "I think film turned [for me] after I did that," she says. "When Capote happened,although it was a small part, I thought it was a great part, and it whetted my appetite for more. I started reading scripts differently, and said, 'No' to the ones where you're pretty much just standing around – like in War of the Worlds."

Now it feels like Ryan is finally reaping the benefits. Like Laura Linney, another New York theatre actress who broke into film late after an Oscar nod, Ryan is suddenly on everyone's hot list. Next week, at the Cannes Film Festival, she will be seen in Changeling, the new Clint Eastwood film, alongside Angelina Jolie. It's another story about a kidnapped child – this time a true one set in the 1930s. Jolie plays Christine Collins, a single mother who went up against the corrupt LAPD after the disappearance of her son, with Ryan cast as a woman "who befriends her and helps her through one of the darkest hours that she faces".

Ryan is completing Green Zone, by United 93 director Paul Greengrass. A CIA thriller set in 2003, during the Iraq invasion, it follows two agents (Matt Damon and Greg Kinnear) tracing weapons of mass destruction. Ryan plays a Wall Street Journal reporter following their mission.

Despite her recent rise, she's not bothered by the paparazzi. "I like that," she says. So, while she may not be taking her own laundry to get cleaned for much longer, Ryan seems too wise to see her head turned.

'Gone Baby Gone' opens on 6 June