Some consider Rosmersholm to be Ibsen’s masterpiece. Yet the work is seldom staged, despite the fact that Ibsen is the second most performed playwright in the world after Shakespeare. And it seems no one can pronounce the play’s name. Is it Roshmerschlum? Roshmerbosh? Rumblebum? Rashmagash?

When the offer came in to play the drama’s heroine, Rebecca West, the first image that came to my mind was of a dusty parlour made entirely of brown carpet. I expected a bum roll under a crinoline and a lot of moping in metaphors. So how am I now here at the Duke of York’s theatre, sitting on the bed in Dressing Room One (which has a bed in it, thanks to Ian McKellen) and performing this play eight times a week to a packed house?

Well, I read the script and was captivated. I had no idea what would happen at the end, but when it came it felt at once satisfying, inevitable and mysterious. Duncan Macmillan, its adapter, had dusted out the parlour, stripped the brown carpets of baggy exposition and transposed Ibsen’s big ideas into something accessible to the modern ear. “This is what happens when the general public becomes engaged with politics,” says the conservative Governor Kroll. “They get duped into voting against their own interests. They’d be better off leaving it to the people who know what they’re talking about.” Our audiences audibly gasp in recognition of the arguments and ideas that are all too familiar in today’s political climate. These are told entirely through passionate, personal relationships. The integrity of Ibsen’s story and its themes remain potent, its metaphors vivid and chilling. Lucy Briers, my co-star, does wear a bum roll, however. Personal choice.

Firstly, for clarity, it’s Ros-mers-home, which translates as, well, Rosmer’s home. In our version it is the eve of a general election and there are many undecided voters. Kroll visits the great house of John Rosmer, former pastor of the town. As a man of influence and power, Rosmer can swing this election – and Kroll, his brother-in-law, is counting on him.

What Kroll doesn’t know is that Rosmer is undergoing a crisis. He has expunged his faith, his background, his beliefs. He now wants to use his privilege to help free society from the corrupt political elite and the dead hand of the church. But in doing so, he has been left with a vast, existential hole. What can he fill it with? If God is dead, who can Rosmer turn to? Himself? Who’s that then? He is both lost and highly suggestible. Underpinning this is the insurmountable wall of grief he is battling after the suicide of his wife – Kroll’s sister Beth – a year earlier. The riddle of her death still holds the living captive.

‘The arguments are told through passionate relationships’ … Atwell with Tom Burke. Photograph: Johan Persson

Enter Rebecca West, former companion to the tragic wife, who has curiously remained in the house since. Eyes blazing, heart on fire, West is a radical feminist, a self-educated activist, a woman who desires agency in the world but has none. What she can do is quietly influence Rosmer, a vulnerable man, and gently guide him towards the growing movement of emancipation as the old world disintegrates.

She is Kroll’s worthy adversary in a war of the worlds – and Rosmer is the territory they battle over. Kroll, a reactionary, can see that Rosmer and Rebecca are in love, and he begins to suspect that a darker narrative drove his sister to suicide. This suspicion could undermine Rebecca’s mission entirely. Who will win? Who is the saviour, who the destroyer?

It’s not exactly a farce. But there are a lot of comic moments in it, which we only truly discovered when we put it in front of an audience. You only have to look at our production from the viewpoint of the housekeeper, the admirable Mrs Helseth. Dismayed at all the interruptions to her rituals, she tries to keep the household intact and its inhabitants within their allotted roles, thank you very much. In quick succession, a room that has been locked for a year is broken into without her consent; a man in a large hat arrives, throwing the staff into a frenzy of activity; a vagabond she thought was dead turns up unannounced to set the world alight with his “visions” of “the truth” – and eats a lot of fish. By act two, everyone is suddenly in the bedroom, and then Rosmer has what we like to call a “Morrissey moment” with a bunch of flowers, declaring earnestly that he has a great capacity for joy. Ibsen, you fiend.

When it was first performed, we can only assume there was a very different response to the character of Rebecca. The audience would naturally have identified more with the conservative, middle-class Kroll and been quick to denounce this ambitious woman as a manipulator and seductress. At one point, Kroll even says to her: “You are unable to feel. You have a cold heart.” Rebecca, meanwhile, talks of love, freedom and joy, while fighting for the rights of the working classes and women. Far from cold, she’s ablaze, underlined by the pervasive use of fire imagery in her language. In the first half of the play, her male counterparts routinely interrupt her. But instead of allowing this to diminish her, she ensured it propels her into a course of action that initiates the beginning of the end.

It is hard for me to write about Rebecca. I live her every night and yet she keeps slipping through my fingers, as mercurial on the page as she is on the stage. I have come to love the mystery of her. At first it was frustrating to try to work out why she does what she does, why halfway through the play she confesses to something in a way that seems entirely out of character. Is she lying? Is she telling the truth? Even if so, is it machiavellian or an act of mercy? Each version can take dominance on any given night and, as the actor, I don’t know what causes one particular shade of her to emerge. Some nights, it’s all three.

‘Leave it to the people who know what they’re talking about’ … Giles Terera as Kroll in Rosmersholm. Photograph: Johan Persson

I was on my way to the dressing room yesterday and a man stopped me at the stage door. He seemed perplexed and intrigued. He’d seen the show the night before and had come back for answers. He wanted to know if my character was really telling the truth. Rather frustratingly for him, I just replied: “It’s up to you what you make of her.” He looked at me in exactly the way Rosmer and Kroll look at Rebecca as they say: “I don’t know what I’m looking at.” It’s become my favourite line.

Rebecca can hold contradictory qualities that are more often seen in the heroic male roles of classic theatre. She lives out internal battles between personal freedom and sacrificial love. She’s torn between the domestic realm and an epic call to adventure. She’s so psychologically complex Freud wrote about her in an essay. Today, audiences are finally willing to see her as a fully formed heroine, beyond the limitations of her social circumstance and implied dark past. When the lights go down and I’m wiping snot from my face and on to the tattered remains of my once-binding costume, I feel a sense of release and relief. Enthralling or appalling, she demands to be seen and heard. She is a heroine for our times.

At the Duke of York’s theatre, London, until 20 July.