FATHER and son George Costigan and Niall Costigan are recalling the first time they worked together.

That was in the British film Bob, Sue And Rita Too, getting on for 30 years ago.

Niall wouldn’t actually call it work as he was six or seven at the time. “Every time I watch it, you get smaller and I get younger,” says George.

“What do you remember about it?” he asks his son.

“I just remember messing about. I enjoyed it, but got told off at one point because I looked at the camera. I got paid £100 and it was all very liberal in our house, so I had to share the money.”

Since then, the pair have worked briefly together on a reading of Shakespeare’s King Lear at York Theatre Royal, where they’ve returned to play father and son in Caryl Churchill’s play A Number. To be more accurate, father and sons as the piece explores the subject of human cloning and identity.

George as the father and Niall as his three sons, two of whom are clones of the first one.

By coincidence, George has three sons. “And very different,” adds Niall, the only actor among them. One of his brothers teaches film and directs, the other is in a band.

Director Juliet Forster had the idea of putting them together in the play. Both have worked at the York theatre before – George in Death Of A Salesman and Blackbird, Niall played King Arthur in last summer’s big family show. “She said she wanted to do it with the two of us and if we didn’t want to, she wasn’t that interested. She kept on hearing us when she read it, so it was a lovely email to be asked to do that. Very flattering,” says Niall.

Having a real father and son replicating that relationship on stage obviously has some impact on the production. “I can already feel there’s a huge advantage because you’re not going to doubt for a second when I say to him ‘I’m your father’,” says George.

“Niall saw the play with Michael Gambon and Daniel Craig and there’s not as much connection between them as between Niall and me. There can’t be. They might be better actors than us, but on that single level we’ve got them.”

We spoke in the early days of rehearsal when their real-life relationship hadn’t affected Niall’s thinking yet. “We’re still just trying to figure out this maze of a world that Caryl Churchill has created,”

he says. “Where the characters start and where they go to is so far-fetched that there’s nothing I can use to recall that. The father and son thing helps because we can sound very much alike if we choose to and kind of look alike. Physically, there might be some traits other people will pick up on better than me.”

Telling people what the play is about can be tricky, “I’ve tried to describe it about 15 times and each time I confuse myself,” says Niall, who tries anyway.

“It’s about a father who has three sons – one that he raised, one that he got rid of and one he has no connection with whatsoever.

The father had his first son cloned, got rid of the first one and started again with the second son. Because this is a play about cloning there were 20 other clones made. The **** hits the fan when they find out.”

George feels the play might be a tricky sell to audiences, “I go to where I always go now, this is just great writing. As with the best of authors, you come out full of questions, full of curiosity,” he says.

With a recent TV programme showing dog cloning, perhaps cloning humans isn’t too far away. “The father in the play tells his son, ‘I had you cloned because I thought you were perfect – I didn’t want another one, I wanted the same’.

That’s not hard to get to at all. Any parent would feel that, especially about their first born.”

So how is working together? “It’s certainly different from working with another actor because if someone else was playing my son I’d be putting in a great deal more thought and effort into how I touch him, all the sort of things I’m not giving a second thought to,” says George.

“When I did Death Of A Salesman, I had to make a relationship with both of the actors playing my sons. Actors are used to it, it’s part of the process. You make a deliberate effort to be fond of them because they’re your sons. But it’s not the same as working with your own flesh and blood.

“You have a shorthand if you’ve worked with people before.

I have more than shorthand with him and he has with me too.”

Having an actor for a father and a director for mother had an influence on Niall going into acting. Although his theatrical beginnings were in France, where the family moved when he was young. “I went to a great college in France that had a great theatre course and also the biggest theatre festival in Europe,” recalls Niall.

“The French think very differently.

He thinks very differently to the way I do,” says George. “He’s not only an actor, but also directs. That’s the bit I’m looking forward to because he thinks in a different way to me.”

From that remark you’d gather correctly that he’d love to be directed by Niall. The father and son relationship could well be headed in another direction after A Number.

  • A Number: York Theatre Royal Studio, May 1 to 24. Box Office: 01904-623568 and yorktheatreroyal.co.uk