THE seeds were sown when actor and theatre director Samuel West made a flying visit to York Theatre Royal last autumn to promote the My Theatre Matters campaign in support of keeping regional theatre alive.

"When Sam was here he mentioned this play he'd done with his dad [Timothy West] while he was artistic director of Sheffield's Crucible Theatre," says Theatre Royal associate director Juliet Forster.

"The idea of George and Niall Costigan doing A Number began to form in my head as I read the play, and I said that if I could get George and Niall to do it, then I would definitely direct it as they just seemed the perfect fit."

Move forward to April 2014 and Costigan senior and junior are rehearsing Caryl Churchill's two-hander for its York premiere, performing together in a play for the first time, having both appeared in past Theatre Royal productions; George in David Harrower’s Blackbird. Niall in the lead role in Mike Kenny's The Legend Of King Arthur last summer.

More of the father-and-son partnership in next Thursday's What's On, when the Costigans will discuss their roles.

First, director Juliet considers the continuing impact of Churchill's 2002 play about human cloning and identity.

Set in the near future, A Number focuses on the conflicts and relationship between a father and his three sons in their mid-30s, two of whom are clones. Through the eternal debate on nature versus nurture, Churchill asks: imagine if you found out one day that you were only one of a number of copies, how would you feel, and how would you know if you were the "real one", the human one?

"This is my first Caryl Churchill production, and it’s one of my favourites," says Juliet. "In her plays she manages so brilliantly to capture a range of big political ideas often within a tiny, domestic setting, and here she's exploring human ethics and identity in a thoroughly relevant and contemporary manner.

"What Caryl Churchill does is take a big political issue and encapsulate it in a father-and-son relationship, where there's parental guilt, sibling rivalry and a loss of individuality. All this is crammed into an hour-long piece, but I don't think you could cope with more than an hour. It 's a short play, but it leaves the mind full."

Juliet notes how Churchill does not complete the story.

"The play raises the issues, the big questions, and it's left open-ended but you don't feel cheated by that," she says. "I don't think there's a conclusion to it because, on the surface, it's about cloning, but the play doesn't massively delve into that because the consequences are so clear.

"Caryl Churchill looks at how there's something about losing our individuality that causes fear in us, while at the same time she celebrates our differences, despite our similarities, as we're all unique. It's much more a play about what makes us who we are and what matters in our individual lives."

Juliet hopes we would always react against homogenisation.

"But you go through a phase as a teenager where we want to be like each other, until you're in your 20s when you're forever trying on different masks and discarding them to work out who you are," she says.

She suggests Caryl Churchill leans towards nurture in the Nature vs Nurture debate.

"She thinks 'nurture' is more important, and that's true of genetics too, where we used to think that DNA established everything but what research has shown since the 1990s is that genes are changed by their environment," says Juliet.

"It's not the preordained blueprint that we used to think it was, and there's a lot in the play that's trying to excuse behaviour, but so much more of it is to do with our environments."

Nature and Nurture come together in any father-and-son relationship, so having George and Niall Costigan in the production adds to the intrigue of the piece.

"The play has had an interesting track record of fathers and sons performing it, but I wasn't thinking in terms of that factor when I cast George and Niall," says Juliet.

"I was thinking in terms of their voices and the great depth that can be brought to the play by their physical similarities, their mannerisms, and that definitely gives the production another layer and an extra dimension."

York Theatre Royal presents A Number, York Theatre Royal Studio, May 1 to 24, 7.45pm plus 2.30pm, May 8 and 22, and 2pm, May 17 and 24. Box office: 01904 623568 or at yorktheatreroyal.co.uk