A night on the sofa in front of the telly will never be quite the same again. For when you share your night on the sofa with a couple of hundred playgoers on the converted stage of Northampton’s Royal Theatre, it takes on a whole new dimension.

This second production in the company’s Alan Ayckbourn season to mark the playwright’s 70th birthday throws out the theatrical rule book in a bunch of different ways. Written in 2004 to a self-confessed filmic style, the play consists of 54 scenes in which the intertwined lives of six lonely people unravel to the point where only one relationship – a brother and sister – remains even remotely viable.

All of which makes it sound relentlessly miserable. It isn’t. Thanks to the intelligent crafting of its author, the barbed one-liners and the painful humanity at the heart of every character, there’s plenty of warmth and wit to offset the poignancy.

Artistic director Laurie Sansom – a one-time Ayckbourn apprentice at Scarborough – serves his former master well with this version, only the third professional production to be staged in the UK. Closing off the Royal’s auditorium, he instead places the audience in the thick of the action itself, using the full width of the wings and depth of the stage to create performances spaces from pools of light and suggestions of furniture. The actors wander among the viewers, playing out their dramas up close and personal, often sitting or standing just inches from the voyeuristic audience. It’s not so much theatre in the round as theatre in the altogether.

There are problems with this. While I enjoyed the luxury of a saggy settee, others struggled through the two uninterrupted hours perched uncomfortably on bar stools or shifting buttocks awkwardly on scatter cushions. Sight lines are sometimes impossibly difficult, and the height of the fly tower above occasionally soaks up actors’ voices, especially when they’re facing in the opposite direction.

But all that is ultimately blown away by two things: the boldness of the whole project, and a set of uniformly superb performances.

Sansom’s ambition continues to vault far beyond the apparent confines of his little Victorian playhouse, and the vision and flair with which he abandons the Royal’s physical conventions matches the vision and flair of the play’s author, who observed the press night from a balcony high above the action.

And both are brilliantly served by their cast, whose dedication to finding the truth at the core of their characters results in some magnificent performances. Lucy Briers combines quiet religiosity with a shocking secret, Matthew Cottle makes his lovelorn estate agent a three-dimensional figure of pity, and Laura Doddington as his sister turns loneliness into an art, while Ruth Gibson and Christopher Harper convey all the acute nuances of a relationship in meltdown. But if anyone could be said to steal the show, it’s Kim Wall as the stoic barman Ambrose. In a remarkable performance, this highly intelligent and creative actor ranges almost imperceptibly from public dependability to private agony in a flawless illustration of the play’s themes.

It’s hardly a feelgood show and you’re strongly advised to get there early to claim one of the more comfortable (unreserved) seats, but even if you end up with a numb bum or aching limbs, there’s no denying Private Fears is another triumph in the Royal’s increasing litany.

- Michael Davies