Never before in US box-office history has a film releasing on as few screens as Precious did last weekend managed to record a higher-grossing opening three days. Despite an R-rating for "child abuse including sexual assault, and pervasive language", Lee Daniels' harrowing drama about the debasement of an illiterate Harlem teenager has earned a massive $1.8m from just 18 selected screens in New York, Los Angeles, Chicago and Atlanta ? a theatre average of over $100,000. It's now opening out in Philadelphia, Washington, Houston, Dallas and San Francisco and Entertainment Weekly has tipped its mesmerising lead Gabourey Sidibe as an Oscar front-runner.

"It's all a whirlwind," she tells me. "I never had any acting training. I was so sceptical of everything to do with the film. This so isn't my life." She was working in a call centre when she got the role. "I didn't want to go to the audition, because I'd have to cut class at college. I was studying to be a research psychologist. But I did anyway."

Having to weigh 360 pounds to play Precious was no problem. "I was fat to start with," she laughs. "I'm never going to be a size two. I'm not built that way. And godammit, I love the way I look."

Her real challenge was to seem downbeat and pathetic. "A lot of the time, particularly at the beginning, Precious wasn't exactly cute. That was the real physical transformation for me, rather than having to be fat. I'm normally a cute girl."

She's amused when people ask if the character stayed with her after the cameras stopped rolling. "I don't know enough about acting to be worried about living with the character. It would freak people out on set because as soon as they'd say 'Cut!' I'd giggle. I'd be myself. Precious is a different person. I was very aware of who she was versus who I am. So it wasn't tough. It wasn't hard to get it out of me."

Sidibe was born in Brooklyn but moved to Harlem when she was eight after her parents separated: her mother is a gospel singer, her father is from Senegal. "I think the film is true to what Harlem is in a lot of ways. That's not to say it's not also universal. It's every neighbourhood. We're drawing attention to a subject that is taboo. There are many people losing their lives to this kind of abuse and other kinds of abuse. It's important that we turn a light on it."

She'd been familiar with the original novel Push by black author Sapphire, but never read it. "I picked it up and put it down at high school so many times because I didn't realise what the book was about. Finally when I actually read it, it was heartbreaking. I know this girl in so many of my friends, so many of my family. I know her in myself. This girl has been neglected and ignored and she reached out for support and couldn't get it. That's what hit me. I felt bad because I've ignored this girl. But I am this girl too."

Getting Precious to the screen has been a long struggle for Lee Daniels. "You're looking at this girl who's abused sexually by her father and physically by her mother, she's illiterate and has HIV and all she wants is to learn how to read and take care of her baby. I don't think even today there's anyone who would take a chance if they heard that storyline."

His music friends Lenny Kravitz and Mariah Carey helped out by appearing in supporting roles, while Oprah Winfrey and Tyler Perry came on board as executive producers. "Although many famous people were offering millions for the film rights to Push, Sapphire gave them to me for a token $1. When I showed her the film she cried in my arms."

Precious has the same electrifying effect wherever it's shown. It wowed audiences and critics at Sundance. Harvey Weinstein and Lionsgate fought over its distribution rights. When I saw it in Cannes, it received a 20-minute standing ovation. Now there's to be a special screening at the White House for Barack Obama.

Daniels produced Monster's Ball, which won an Oscar for Halle Berry, and The Woodsman, before moving from Los Angeles to Harlem in 2004. "I bought a brownstone on 135th Street. I thought since I'm black I should live and work in the black community. The third day I'm there, I go out my front door and cops are all over the street. I call my partner: 'Tell the nanny she's got to stay with the twins, I'm going to be late. There's a drug raid or something.' And there at the bottom of my steps sat Bill Clinton. He was about to speak at a church next door. We got talking. 'I'm here in Harlem, and I want to do a public service announcement,' he says. 'Do you think you can help get young black people out to vote?'" So Daniels did, launching a successful campaign with Rock The Vote, featuring his friends LL Cool J and Grammy winner Alicia Keys.

"For so many years, we black people have had two voices, a voice we had with ourselves and a voice we had for white people," he says. "The moment Obama became president I think all of us lost those two voices and now we have one voice. There are no more pretences, no more bullshit. We are what we are. We're in the White House."

Odds are narrowing on Precious to emulate other low-budget independent films like Little Miss Sunshine, Juno and Slumdog Millionaire by soaring from nowhere to Academy Awards glory, particularly with Oscar PR specialists Cynthia Swartz and Lisa Taback in its corner. "I'm really surprised it got this far," says Sidibe, who's since been deluged with acting offers. "I still want to be a research psychologist. I hope to do more films. But I'd like to run with what I have for the moment and see where it takes me."

Precious opens in Ireland in January