Hathaway portrays Mother Mary, a global star spiralling on the eve of her high-stakes comeback performance following a mysterious incident. Struggling, she travels to England and turns up at the doorstep of fashion designer Sam Anselm (Michaela Coel), requesting her once closest friend who she broke off contact with a decade ago, to make her a dress that truly represents her.
The reunion at Anselm's countryside atelier unearths repressed anger and pain but also reminders of their tight bond.
Hathaway said she drew inspiration from superstar Beyonce and took her training for the film "really seriously".
"I didn't want to just show up and do a good job, I wanted to do my absolute best and it took several years of being really committed to it," Hathaway said at the film's London premiere.
Hathaway said she trained eight hours a day for months to nail a demanding key dance scene and practised consistently while working on other projects, and also spent a year "crafting the voice in a specific way".
The Hollywood star, who a day earlier premiered The Devil Wears Prada 2 in London, said she could understand her character's battles.
"A lot of her is based on the choices that I've actively tried not to make. I'm really lucky I've been in successful films since I was a teenager, but I also knew that I really wanted to be an artist and I never wanted to lose who I was as a person in the limelight," she said.
The 43-year-old said she felt she'd reached an age "where I can appreciate it and enjoy it and not feel buried by it, the way Mother Mary feels buried by it".
The A24 film is written and directed by The Green Knight filmmaker David Lowery and features original songs by Charli xcx, Jack Antonoff and FKA Twigs. Hathaway performs seven songs on the accompanying soundtrack album Mother Mary: Greatest Hits.
"We could have written top 10 hits, but she had to feel them," Lowery said. "She had to sing them and she really pushed to make them as good as they are."
Mother Mary is released in the US and Britain on Friday and Australia on May 14.
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