Very few plays in the modern era hold the kind of intrigue and mystique that Tim Crouch’s Derwen (or An Oak Tree) does. Since its first staging at the Edinburgh Fringe Festival back in the mid-noughties, the play has excited audiences with its highly innovative structure. Technically it’s a two-hander, the story of the meeting between a lowly stage hypnotist and the father of a young girl he accidentally killed. An interesting but unspectacular premise for a production, to be fair. Where the play sets itself apart is in the casting: of the two actors, one has never seen (or read) it before.

Guinea pig for the particular performance I went to see was Emmerdale and Hinterland star Sian Reese-Williams. The actor learns of the story, the character and the mood of the piece in the first few minutes and then she’s off, participant of a play in which she has no preconceptions and, crucially, no control. Therein lies the beauty of Crouch’s writing, though. The character is in as much a state of confusion as the actor and, pretty soon, the lines between reality and fiction start to blur. Every interaction is meticulously scripted, even those moments when the audience are led to believe that the actors are now out of character. The script is ambiguous enough that it keeps both Reese-Williams and the audience on their toes. Her overall performance is by no means a reflection of what she is capable of – it’s a solid but understandably tentative turn, and to expect anything else would be unfair. The joy, ultimately, is in seeing the wheels turn in her mind as she decides how to deliver a line or how to use the space. It’s fascinating.

Steffan Donnelly is the actor stealing the show, however, in a truly remarkable performance. As well as playing the role of the hypnotist, which he does very well, its everything else he does that makes his performance so strong. And make no mistake about it, he does everything else. He is actor, director, dramaturg, set and sound designer. He is Reese-William’s safety net but also, probably, her worst nightmare. Like Reese-Williams, the audience have to place their trust in Donnelly to ensure he gets her, and us, through safely, and that’s unnerving. He is both deity and monster.

The unsung hero of the piece is perhaps director Andrew Whyment. Because of how front and centre Donnelly is, and how much work he has to do on stage, its easy to forget that his performance has also, to an extent, been moulded. Key to the success of the production is that trust in Donnelly, and Whyment lets the actor bring in a lot of his own personality. Or at least, a version of himself that is far more palatable to the audience than the character he is playing. Perhaps accidentally, Mared Llewellyn Williams’ Welsh translation (and subsequent English captioning) adds another layer to what is already a meta production. Non-Welsh speaking audience members are at the mercy of the captioning reinforcing the fact that, like it or not, we are never the ones in control.

Derwen is an enthralling piece of theatre, more for the unpredictability of the performances than the narrative itself. It’s a play that, by its very nature, could never be the same twice, and that’s quite the achievement. Inevitably, audiences will always go away praising the courage of the second actor – and rightfully so – but its Donnelly who truly shines.