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AYLEY ATWELL has described her latest venture as her “most liberating” yet. A notable departure from her portrayal of Marvel’s Agent Peggy Carter, the London-born actress, 36, is set to take viewers by storm as The Long Song’s odious mistress, Caroline Mortimer.

Adapted from Andrea Levy’s award-winning novel, the threeparter – set during the final days of slavery in 19th century Jamaica – follows the trials, tribulations and survival of wily, strong-willed slave July (Tamara Lawrance) on a plantation run by Caroline and her older brother, John Howarth (Leo Bill).

By Hayley’s own admission, it’s her first villainous role to date.

“That was the appeal,” she says of the BBC drama. “Caroline was so big on the page – an almost panto villain, hysterical, grating, heinous monster. But I thought it would be a challenge, for me, if I could create some sort of pathos – or we could see her human side.

“If we saw her desperate and pathetic attempt to control the world’s opinions of her...” she elaborates. “And if we saw what she was like when she was on her own, and it was quite different, that would be a fascinating arc to play.”

For those in need of some background, the best-selling tale begins when soon-to-be heroine July is born to field slave Kitty (Sharon DuncanBrewster). On a whim, Caroline opts to take the young girl from her mother in a bid to keep her as a personal maid in the great house.

Fast forward and July, now a teenager, has learned how to “handle” her mistress. But with the abolition of slavery on the horizon, her destiny is soon thrown into question.

Welcome Robert Goodwin (played by Hayley’s Measure For Measure co-star) – a new overseer, who looks set to revolutionise the plantation once order has been restored. But could this be a turning point for Caroline? That would be telling – but the joy comes from playing a character who is intrinsically hard to like, Hayley insists. “I didn’t like her, but I loved her, and I found her dark side to be an opportunity to explore the damage done to the human psyche when you inflict damage on someone else,” she says. “She knows she’s stupid – and I say that in a sense that she has no kind of real self-awareness and also no position within this circumstance to actually have any true power,” she explains. “She doesn’t own the land because she’s a woman, so she has nothing, and she has no one apart from July.”

Smiling, she adds: “It’s quite obvious how much I enjoyed being her.

“Mahalia (Belo) was so wonderful as a director because she gave us (free) rein to try things – and because I felt safe, it meant that I could take things really far just to find out where the line was.

“Yes, I get to play a ridiculous character, but I was not ashamed to be doing it!” she adds. “I felt very relaxed – and you’ve seen so many of my ugly sides (so now) I feel free.”

It’s a fulfilling moment for the star, who trained alongside the likes of Jodie Whittaker and Michelle Dockery at the Guildhall School of Music and Drama.

“I think, because I’m proud of it, I can now say it has been my most liberating role,” muses Hayley, who is also known for her stage work in such productions as Lindsay Posner’s revival of A View From The Bridge.

“It paid off to be able to steer the trajectory of what I’ve been doing, and what I’ve done before, into new territory,” she says.

“At this stage in my career, this has been a necessary tool for me. I have more ownership of the parts I take, and therefore I can deliver the audience something that they will enjoy watching.”

A rarely acknowledged part of British colonial history, however, how well does the series present the shameful subject of slavery to a modern-day audience?