First, the noise, after a decade of restrained, subtle excellence, Ruth Negga is suddenly everywhere.

During the recent awards season, Negga repeatedly headed those silly best-dressed lists, wearing custom-made gowns from some of the world's biggest designers. At the Golden Globes, she was name-checked by Meryl Streep in her excoriating, Trump-skewering defence of diversity in Hollywood, which is coming up for 9m views on YouTube.

What you probably haven't seen - and must, first chance you get - is the signal, so to speak, which has inspired this fuss. Her new film, Loving, is the real-life story of Mildred (Negga) and Richard Loving (Joel Edgerton), an interracial couple threatened with prison in 1950s Virginia because of their decision to get married. The case went all the way to the supreme court and would become a landmark judgment in American civil rights, although one that was only fully appreciated after Mildred died in 2008.

Negga was one of the standout stars of the awards season: Nominated for a Bafta in the rising star category and an Oscar for actress in a leading role. When you think of awards-buzz performances, it is typically the showy, exuberant ones that catch the eye and grab the lapels. Negga's portrayal of Mildred Loving is nothing like that. It is quiet, even wordless for large chunks.

Loving tells the true story of Richard and Mildred Loving, whose inter-racial marriage was deemed illegal in Virginia.
Loving tells the true story of Richard and Mildred Loving, whose inter-racial marriage was deemed illegal in Virginia.

Emotion is created with weary eyes, pursed lips and a slow, long-suffering gait. Proper acting, in other words. "If Modigliani ever painted the Delta blues, it would look something like Negga's expression in this movie," wrote film critic Wesley Morris in the New York Times.

"It's a thrilling and invigorating thing to discard language in a way, almost completely," explains Negga. "First of all, some things can be really overwrought and overwritten and if it's a realistic story, that's just not the way humans exist: long monologues and quick, sharp banter.

"Then we have the ability to lie verbally but it's much harder to get our bodies to lie, because there's a truth there. We are constantly communicating with our bodies subconsciously and I find that absolutely fascinating. There's a lovely tension that can happen in a silence."

We are speaking on the phone: she has been yo-yoing between New York and Los Angeles and had set aside the week to catch up with family and friends in London before heading off to New Orleans for the best part of six months to shoot the second season of the comic-book TV adaptation Preacher. But, inevitably, people want to talk to her about Loving.

"I have not seen the inside of a yoga studio in too long because of talking to journalists," says Negga, her Irish burr coming as a surprise after seeing her play a run of Americans.

"I've never really had to do anything on this scale before, so it's been a huge new experience for me and I've had some amazing moments. Nothing really to complain about, but my poor old voice is very tired and raggedy."

Negga, to be fair, has gone above and beyond to support a film that lacks the firepower of other Oscar contenders: it was made for just $9m by a director Jeff Nichols (Take Shelter, Mud), who is acclaimed but defiantly indie. The "campaign" started at the Cannes film festival last May, where Loving received a rapturous ovation.

"You hope that you don't let the film down - that's the worry," Negga says. "It's a responsibility, because essentially you're communicating to the world why they should see this film and that they should see this film. So you want to serve it in the best possible way."

Being picked out by Streep is "still surreal", she admits, but it feels like these are career-changing moments for Negga: overwhelmingly positive, though also clearly discombobulating. She has worked steadily since she graduated in 2003, winning effusive reviews in film, theatre and television without ever looking set to become a household name. There has been modest interest in her private life - since 2009, she has dated the actor Dominic Cooper - but mostly she has avoided talking about it and diffused much of the clamour. So does what's happening now feel "brilliant" crazy? Or "strange" crazy?

"It's both those things," she says with a laugh. "I don't think it's just one kind of crazy. You become an actor - some people do, not everybody - to hide and disappear and I worry sometimes, 'Gosh, doing this circuit, as they call it, is very much presenting yourself to the world', and that can be a little intimidating for actors who basically like to hide."

When Negga signed up for drama school at Trinity College in Dublin aged 18, she'd never done any performing at all: no school plays, no terrible teenage bands, nothing. What made her think that she would be any good? "I didn't know I'd be good at it," she says. "I just knew I wanted to do it."

Her inspiration was Kate Bush on Top of the Pops, David Bowie walking down the steps in the 1986 musical-fantasy film Labyrinth. "But it wasn't a lightning-bolt moment," says Negga. "It feels vocational for many actors. Some not, but for some it feels there's a need, a calling. And people might think that maybe sounds a bit wanky, but why should it? Why shouldn't it be like that?"

Negga was born in January 1982 in Addis Ababa, Ethiopia; her father was a doctor at the Black Lion hospital in the capital and her Irish mother, Norra, was a nurse. Ethiopia was a mess then: a civil war raged between the Soviet-backed Derg government and rebel groups and divisions were exacerbated by the droughts that led to the famine of 1983-1985. Negga and her mother left for Limerick when she was four. Her father was supposed to join them, but died in a car accident.

"I didn't go back to Ethiopia until I was 18 and I haven't been back in a very long time," says Negga. "But it's a very familiar culture to me; I grew up eating Ethiopian food and listening to Ethiopian music and reading Ethiopian history, so it's part of me."

But when it comes to being a poster girl for Hollywood diversity, Negga is less comfortable.

"Sometimes it gets a bit tedious being asked about these things, because I can't be a spokesperson for an entire group of people," she says. "It's just like when people call films 'black films' and you're thinking, 'Why?' They are not just for black people, you know. Our film is not about black America, it's about America. White and black America. You know, we're all in this together."

Who: Ruth Negga
What: Loving, the real life story of George and Mildred Loving
When: In cinemas next Thursday