The Legend Of King Arthur runs at York Theatre Royal until August 3

For one night only, in October 2010, he played Oswald to Freddie Jones’s Lear in a rehearsed reading of King Lear to raise funds for the theatre’s Access For All Fund, proceeds from which went towards the theatre’s expansion into the De Grey Rooms next door.

It is in those refurbished rooms that Niall has been rehearsing for his lead role in the Theatre Royal’s family show for Summer 2013, Mike Kenny’s premiere of The Legend Of King Arthur.

It s not for want of trying that Niall has not appeared before in a Theatre Royal repertory production, unlike his father George, who led the cast in Blackbird in the Studio in November 2011 and Death Of A Salesman in November 2008.

“I’ve auditioned for Damian [Cruden, the theatre’s artistic director] three or four times, and this time I auditioned successfully,” says Niall, who joined the company straight from playing Silvio the Shepherd in a garden promenade production of Shakespeare’s As You Like It at Hornchurch in Essex, just off the M25.

This is a contrasting theatrical experience, not least because designer Catherine Chapman is transforming the theatre building into Camelot to enhance “The Legend Of King Arthur Experience”.

In other words, there will be more than a play going on for the audience, with activities ranging from Knight School to heraldry to a chance to eat at the Round Table.

“As part of the ‘event’, the cast will be going round as storytellers for half an hour before each show; we’ll be getting the audience into the idea that we’re going to be sharing stories in the play, including a couple of scenes that are Mummers’ Plays,” says Niall.

“In the second half, anyone who has been to Knight School will get to be knighted by King Arthur, so there are going to be elements that are exciting but also maybe daunting for the audience because we’re expecting a little more from them than usual and wanting them to be involved when the lights are up and when the lights go down.”

Mike Kenny’s play has two matching sets of lead characters, young and older.

“The principle of the show is that young Arthur is a modern-day character who pulls the sword from the stone and is then thrust into this world that he never quite understands,” says Niall, who plays the older version of Arthur.

“The story is told through Arthur’s eyes and in a way it’s a show about growing up, taking responsibility and challenging the notion of destiny: whether you are there to fulfil it or whether you can change it, and ultimately Arthur has to decide for himself what he’s going to do.”

Niall praises the new adaptation by Kenny, the York playwright who already has revitalised The Railway Children and The Wind in The Willows, as well as writing the 2012 York Mystery Plays.

“There are so many versions of these Arthurian stories, so what Mike has done is take the legend and find the heart and soul of the different versions, while asking ‘Do we make our own choices or are they made for us?’” he says.

“A conventional telling of the story of Arthur it is not! It’s challenging people’s perceptions of seeing a show, because they’ll experience something different.”

Niall is enjoying the “safe feeling” of working with Kenny and Cruden on a premiere with all the unpredictability that entails.

“You never feel they don’t know what they’re doing, and that’s greatly reassuring when it’s a premiere where we’re all keen to see how it works out,” he says.

“As an actor, you don’t often come across experiences like this, so you really want to savour every moment.”

• The Legend Of King Arthur runs at York Theatre Royal until August 31. Box office: 01904 623568 or at yorktheatreroyal.co.uk