In reality, Elizabeth I never met Mary Queen of Scots. Neither did the two of them share a confidante in Robert Dudley, Earl of Leicester. But in the theatre things are different, and the centrepiece of Friedrich Schiller’s magisterial drama Mary Stuart is a confrontation between the two queens, in which French Catholic Mary pleads with English Protestant Elizabeth to lift her death sentence.

It is riveting writing, but in the latest National Theatre of Scotland production, the joint stars stay truer to history than fiction. It’s nothing personal, say Siobhan Redmond and Catherine Cusack, but, until the play opens in Glasgow, they’re keeping their distance.

“In rehearsals, it did feel like we were in two courts,” says Cusack, who plays Mary. “We’d end up at opposite ends of the table.”

“We’re not sharing a dressing room either,” says Redmond, who, despite her Glasgow pedigree, is playing Elizabeth. “When Mary and Elizabeth are alike, they’re very alike, but where they’re not, they’re like women from different planets. They can’t understand each other at all. So we haven’t spent any time together because it’s not helpful at this point.”

It is rare to hear such uncollegiate talk from two actors as they sit side by side on the same couch, but the theme and structure of Schiller’s play makes it possible. Apart from the pivotal scene, the two queens live in separate worlds, Mary holed up in Fotheringhay Castle and Elizabeth holding sway at court in Westminster.

“I haven’t seen most of what Catherine does in rehearsals — I’m just seeing it now we’re running the whole play,” says Redmond, wearing a dressing gown after removing her voluminous dress during a break at Glasgow’s Citizens’ theatre. “I like Catherine very much. Roll on the day when we’re secure enough to spend time together.”

Echoing the sexual rivalry between the two queens, Redmond looks over at Cusack in her loose-fitting rehearsal clothes and makes light of their 10-year age difference. “It feels like Brazilian drag queen versus child bride,” jokes the 47-year-old.

“That’s not true,” counters Cusack, the London-born daughter of the late Irish actor Cyril Cusack and half-sister to Sinead, Niamh and Sorcha. “My brother was taken away by how stunningly gorgeous Siobhan looked this morning.”

“But you look seven,” persists Redmond. “I’ll be had up for child murder!” Some would have liked the National Theatre of Scotland to make a flag-waving statement in its inaugural year, but Vicky Featherstone’s production will confound the nationalists at every turn. This German play, translated by Glasgow’s David Harrower, about a key moment in Anglo-Scots history, stars a Scottish actor as an English queen and an Anglo-Irish actor as a Franco-Scottish queen. Like the power politics of the play, which gives a forceful voice to both monarchs, the national politics of the production are grey, not black and white.

“I was thinking about my short hair and the fact that I’m from down south via Dublin and then I thought, well, it’s not a historically accurate play,” says Cusack, who last worked with Featherstone on The Glass Menagerie in 1994. “Schiller’s Mary and Elizabeth are up for interpretation by any number of actors in lots of ways.”

“I’m expecting to have to get an armed guard to the station,” says Redmond, aware that playing Elizabeth in Glasgow and Edinburgh will not make her the people’s favourite. “It’s brilliant Catherine is playing Mary, because if it had been a Scots woman, people would have complained ‘She should have come from Tillicoultry’ or ‘She should sound French.’ Yet Catherine has a slight otherness to her voice that you can’t quite place, but is very alluring and absolutely perfect.”

Coming from an acting family — one sister, Sinead, married to Jeremy Irons, the others stars of Heartbeat (Niamh) and Casualty (Sorcha) — Cusack was wary about going into the profession, but slowly discovered it was where she felt most at home.

“It took me a long time to realise it,” she says. “I drifted into it without great will and determination to be there. I was really quite pathetic when I was young. I didn’t have a big burning flame leading me from inside. I did a year of a drama degree to cement in my brain whether I wanted to do this.

I started working and just carried on, and I guess it’s in the last 10 years that I’ve realised I have the best job in the world.”

Fame is a by-product that puzzles them rather than irritates, and they’ve both had similar tastes of it. Cusack played Carmel, the nanny from hell, in Coronation Street and, more recently, the policewoman Frankie Sullivan in Ballykissangel. Redmond, meanwhile, has been seen in Sea of Souls, The Catherine Tate Show and EastEnders as Maeve Brown, wife of Victor, patron of the Walford Community Charitable Trust.

“People would think I was a nurse who’d nursed them,” says Cusack, remembering the aftermath of her stint on Coronation Street. “I got some scary letters and some nice letters, but it didn’t seem to last very long.”

“I played a paediatrician [Janice Taylor in Holby City] and it took me ages to work out why people were telling me about their hips,” says Redmond, then adds: “The High Life is available on DVD now, so I still get an extra sausage on the plane because of playing Shona Spurtle, the stewardess.”

Cusack chips in: “My dad played a cardinal in a film in America, and I got to go for a couple of weeks — very exciting. They did a big scene in which extras had to go by and kiss his ring. A woman from the crowd came up and asked him to bless her. He tried to explain that he was an actor and she said, ‘I don’t mind, would you bless me?’ So he did it.”

Rehearsals beckon. It’s time for the two stage queens to leave the room. One at a time. “Thank you for introducing me to Catherine Cusack,” quips Redmond as she heads to her dressing room alone.

Mary Stuart, Citizens’ theatre, Glasgow, October 3­21; Royal Lyceum, Edinburgh, October 27­November 18

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