FATHER and son George and Niall Costigan have appeared in a film together, playing father and son Bob and Simon as it happens, in Rita, Sue And Bob Too in 1987.

However, they have never a shared a stage, until tonight, 27 years later, in Juliet Forster's repertory production of Caryl Churchill's cloning drama, A Number, in the York Theatre Royal Studio.

Once more, they are cast as father and son, or rather as father, son and two clones, with the last three played by Niall, in Churchill's exploration of human ethics, cloning, identity and parental guilt set in the near future where relationships are riven by conflicts.

"Imagine 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?" asks Churchill.

George first had the chance to appear in A Number seven years ago in Dublin, when invited to step in at short notice at the Gaiety Theatre after an actor dropped out. George mulled it over, but the circumstances were not right, "though you don't ever want to say 'No' to a Caryl Churchill play, as to most actors she means utter quality".

"I'm always interested in doing her plays, like I'm always interested in doing Arthur Miller's plays, and this time I could say 'Yes' to Juliet," he says. "The fact that Niall was going to be in it too made extra sense."

Now they are rehearsing Churchill's script, George can see why father and son Timothy and Samuel West were drawn to performing A Number a number of times. "If I met Tim or Sam, I know what I'd say. 'Tell me what you found in it each time'. 'What did you find you'd got right or wrong?'. 'Would you do it again?'."

You sense that even before tonight's opening show, George and Niall would happily sign up at the double to perform A Number again.

"Caryl Churchill gives you so many different options when you're working on the narrative structure in the rehearsal period," says Niall.

"For us, with the way the characters work, there are three or four different thoughts with each line or syllable. So we've tried to layer the meaning of each line, but that also means that the words might trigger something different on a particular night because you pick up on something else."

"This stuff is so detailed, but the script is also littered with sentences that have no ending or are left hanging," says George. "So there are clues, or that is what we think they are, but then actors are always detectives anyway. They're forensic."

Niall is finding working with his father in the "dad and his lad" two-hander a most revealing experience. "What's interesting is watching his mind at work, watching him work out his performance, trying out what he's going to do on stage," he says. "I've never witnessed that first hand before."

Caryl Churchill's A Number runs at The Studio, York Theatre Royal, from tonight until May 24. Box office: 01904 623568 or at yorktheatreroyal.co.uk