The English actor was nominated for Michael Morris’s directorial debut To Leslie after the film received an unexpected, late surge in celebrity endorsements.
In the movie, Riseborough, originally from Newcastle, plays an alcoholic single mother from West Texas who won and squandered $190,000 (£154,000) from a local lottery.
“I’m astounded,” Riseborough told Deadline in reaction to her nomination.
“It’s such an unexpected ray of light. It was so hard to believe it might ever happen because we really hadn’t been in the running for anything else. Even though we had a lot of support, the idea it might actually happen seemed so far away.”
To Leslie was given a limited release on 7 October, grossing just $27,000 (£22,000) at the box office.
It had almost entirely flown under the radar until a few weeks ago when stars including Gwyneth Paltrow, Kate Winslet, Charlize Theron and Jennifer Aniston began voicing their support for Riseborough’s campaign on social media.
“The response we’ve had all along has been so personal from people,” Riseborough said.
“You judge Leslie in some moments, you identify with her in others, and you go on so many journeys. It felt like there was a scope for it to touch so many people. It felt like the most natural thing in the world to do, to keep talking about it even if people weren’t very interested in it because as soon as they watched the film, they became instantly interested and couldn’t shake off the story. The film really drove itself in that way.”
Bar an Independent Spirit Award, the film had only received a small smattering of nominations this awards season ahead of the Oscars.
“I’m not entirely sure how the f*** this happened,” Riseborough continued to Deadline.
“After SXSW there was a quiet lull. And then slowly, as the film had a few screenings elsewhere – including at Raindance, which was a big deal because we hadn’t had a release in the UK – we found people were starting to talk about it. And people were asking us: ‘Why can I not go and see it? Where can I see it?’ After a while, we were able to point them to iTunes and Amazon, but it didn’t happen right away.”
Another dark horse among the nominations this year was Paul Mescal for his role as a young father in Charlotte Wells’s directorial debut, Aftersun.