In Lies We Tell, Agnes O’Casey plays Maud, a recently orphaned heiress who was raised by her father, away from society. Independent, opinionated and determined, her strength is tested when her uncle Silas (David Wilmot) arrives at the manor, claiming the property and her inheritance, and using his violent son to try and coerce Maud into surrendering everything. The atmospheric thriller turns into the story of a gaslit woman’s fight for freedom from the oppressive and vicious sexism seeking to tear her apart. It’s gritty, propulsive and atmospheric, and O’Casey puts in a superb performance.

“The thing I liked about her is that she’s been brought up basically with just her dad, and she’s lived a very secluded life that she’s really enjoyed,” says the actress. “She’s an intellectual and she’s been allowed to just be and enjoy the library, which a lot of women weren’t. Also, she’s kind of been hidden from society, so I always got the feeling she doesn’t have a sense of the normal programming of what it means to be a woman in society - in a great way.

“There’s a Margaret Atwood quote, ‘You are a woman with a man inside watching a woman. You are your own voyeur.’ But Maud doesn’t have that man in her head, she’s free of that. I found that really fun, because I’d never got a chance to play someone who just doesn’t think about herself in those ways, about having to be beautiful or docile or ‘feminine’ in expected ways. Then it becomes really interesting when Uncle Silas comes in, because he’s not used to meeting anyone like that. He’s really good at the game of manipulation - but she doesn’t play by the rules and isn’t predisposed to being manipulated. She sees things clearly and addresses them which is fascinating.”

The film deals with heavy themes, but O’Casey hopes the audience appreciates the resilience of her character.

“The energy of the film is about a woman who won’t stop fighting, so I hope audiences enjoy the tension and catharsis.”

O’Casey had worked with director Lisa Mulcahy before on the BBC One show Ridley Road, which saw the actress play a Jewish girl living in ‘60s Soho, who agrees to infiltrate a neo-Nazi group to fight against the rising fascism of the time. It was O’Casey’s first television job and she received rave reviews. The actress’s grandfather was Jewish, and getting the part encouraged her to look into her family history. She was also very aware of the political resonance of the show for modern audiences.

“I felt really excited that my first job was something that really resonated with me politically, that was a thrill,” says O’Casey. “But there were some hilarious right-wing responses to it (laughs). Men on Twitter were like, ‘How dare they criticise the Conservatives like this?’ And it was like, ‘We never said anything about the Conservative Party. It’s interesting that you’re seeing them when we’re looking at Nazis. That’s obviously coming from you.’ That was so funny and quite illuminating.”

O’Casey is also starring in the upcoming comedy The Miracle Club with Maggie Smith, Kathy Bates and Laura Linney, about a group of women who win a pilgrimage to Lourdes. For O’Casey, the opportunity to work with such powerhouses was a dream.

“It was the most surreal experience in my life,” she reflects. “I was so intimidated, but immediately they were all so nice. Laura Linney would come around and do laundry at mine and chat. Maggie is so funny and lovely, and very protective, looking out for me in scenes and being so supportive. And they have so many amazing stories. I was trying to be subtle and not ask too many questions - but then she would just casually say something about what Jane Birkin was like.

“And you’d be like, 'Thats just the coolest thing ever!’ Then Maggie would say something really profound and I’d say, ‘Oh that’s so beautiful, did you come up with that?’ And she’d reply, ‘No, that’s Wordsworth’. You can’t even be embarrassed - you got corrected by Maggie Smith. The dream!”

Lies We Tell is in cinemas from October 13.