On the terrace of the Barbican Centre, the wind blows decisively. The sky shifts, throwing shadows over the sun in unpredictable waves. Niamh Cusack brings the material of her cardigan closer to her skin, her hair joining the elements in a brisk dance. It is the first day of rehearsals for the RSC’s transfer of Macbeth. Having moved from their home in Stratford-Upon Avon, Polly Findlay’s adaptation is set to sew tragedy into the seams of London’s most brutal theatre.

As Lady Macbeth, Cusack plays alongside Christopher Eccleston in the title role. She laughs, her brows knotting together before she speaks. “I suppose where I start from in any part I play is that I imagine myself and the character as two babies that have been swapped in the cradle, so I got Lady Macbeth’s history and she got mine.” Her green eyes are joined by a smile. “You have to use yourself. I’m a 58-year-old woman, I’m past child bearing age – these are things that have really fed into the production”. Exploring Lady Macbeth’s past experiences of miscarriage as well as the loss of a young child have been key for Cusack in terms of approaching the performance. For her, tragedy lies in the couple’s struggle with grief.

Along with the undoing of her marriage, bereavement is paired with loneliness. “I think [the character of] Malcolm is wrong in calling Lady Macbeth a “fiend-like queen.”’ Cusack says, her gaze unfixed. “the [Macbeths] are flesh and blood, they’re not monsters”. The pain that they experience is indeed humanising, and she rejoices in the raw immediacy displayed by her onstage partner: “I think we are complimentary together. I’ve always been a more obedient actor, and he is wonderfully anarchic”.

Having worked with Findlay twice previously, Cusack notes how in classical works she likes to make stories recognisable. The cast are in modern dress and operate within a “spare set”, with the action split across two separate floors. At the point of Duncan’s murder, a digital clock glares red in the back, counting down to the end of the play. In this, time becomes tangible, its presence weighted by the fact that Macbeth is Shakespeare’s shortest tragedy. There is a constant sense of being propelled forwards, and the speed of the action has surprised Cusack: “there is no warm-up for Lady Macbeth.” She is right – her fellow actors have a more measured journey to the summit, while her role demands the opposite trajectory.

“It is very exciting” she says, a hand flying out to save her peppermint tea from the sudden breeze. She admits that it is intimidating though, given how many brilliant actors have taken on the part previously. The banquet scene in particular has been most challenging – it is a monumental balancing act. “Covering for [Macbeth], being furious with him and retaining some kind of status is a hard thing to do” she says, after some consideration. It seems that this moment in the play also highlights how fractured their marriage has become, and holds a realisation for Lady Macbeth in that she is no longer able to influence the actions of her husband. The space between them begins to ache, and repeated contact with the supernatural becomes a symptom of the loss that they share.

Concerns with the uncanny also translate into Findlay’s directorial decisions. In choosing to present the three witches as young children, she has magnified traces of horror that are present throughout the production. The Porter too, appears once evil has been introduced, as an omnipresent reminder of darkness. According to Cusack, many of Findlay’s creative choices are fuelled by the horrific nature of murder, war and the search for power. The creative team behind Macbeth is also female-led, and Findlay has doubled the number of roles for women within the production, with the original boasting only two. The Doctor represents one of these additions, while Lady Macbeth’s servant and Donalbain are subject to multi-roling.

For Cusack, the Bard has always been irresistible. In this, she shares her concerns about the current climate of texting and emojis. “We are in danger of losing the ability to articulate how epic it is to be a human being.” She frowns, with spray from a nearby fountain threatening to come our way. Cusack is no stranger to the power of Shakespeare’s pen, and it is precisely her love for his sense of language that has enabled her to connect with his stories. She remembers Giles Block of the Globe Theatre – once with the title ‘Master of Words’ – and how for her, he helped to erase the language barrier that surrounds Early Modern English. His concentration on the spaces between the text, as well as the necessity of the text itself is useful, especially for fledgling readers.

Her love of poetry has also contributed to her relationship with Shakespeare’s work. “His plays are relatively simple, but the way in which his characters express themselves is so rich”, she says, passionately. She maintains that this is what makes his writing accessible, and urges young creatives to align themselves with the power that hides on the page: “just reading it isn’t enough, you have to speak it”. Her Irish lilt carries across the air, now increasingly cool. “There is a need to speak, and we need this now more than ever.”

Macbeth is playing at the Barbican until January 18, 2019. For more information and tickets, see website.