Donnacadh O’Briain and Kefi Chadwick are an interviewer’s dream: they talk, freely, over each other, to each other, to me. They interrupt each other, back each other up and develop each other’s points. They are unperturbed by the small, pink-eared mouse that shares the cafe we’re talking in. It’s clear after a few minutes in their company that they have an enviable working relationship, and enjoy and respect each other’s company. When we meet, director O’Briain and writer Chadwick are about to open their production of Mathematics of the Heart at Theatre503. Despite having just finished their penultimate day of rehearsals, and having had to create a new character at the last minute after one of the cast hurt his back, the two are relaxed and cheerful, and infectiously excited about the play.

In O’Briain’s words, their production will have “the biggest design that the theatre has ever had… It’s 360 degrees, wallpapering the entire 503 –  the whole theatre becomes a sitting room, the tech box becomes a toilet, there’s a fake ceiling with skylights and rain that rain on it… we’ve got stars, a front door…” He trails off, sounding like a kid in a sweetshop, and Chadwick chimes in: “It’s as  immersive as we can make it, without taking the seating out!” 503 may be brave, welcoming and creative, but one imagines that they would baulk at structural damage…

So, how have the two tackled such an ambitious goal while developing a fruitful working relationship? Isn’t it tricky for a director to work with the writer in the room, and vice versa? Apparently not, in this case: “The best note I’ve ever had about working on new writing was from Dominic Cook,” says O’Briain. “And he said, ‘working with writers is the same as working with actors’. Having trained as an actor, I am a very actor-centred director – I tend to put myself in things and find my notes from inside rather than outside. That’s probably why we were able to work quite intimately together. Kefi has the characters very close to her and I come to it with an actor’s point of view, so I look from inside the characters as well. Our discussions have tried to get quite into the meat of what’s going on.” Chadwick agrees: “If we hadn’t worked together before I’d be really nervous, but he has a lovely imagination for solutions.”

O’Briain and Chadwick are passionate about what they’re trying to do. As O’Briain says, “I don’t want people not to care. One of the characteristics of modern, full-on fourth wall drama, at the extreme end, is that in essence it doesn’t care what the audience is doing or how they’re listening. Actors care deeply, but the style of full-on fourth wall naturalism is inherently alone in its space, and the auditorium is alone in its space, too. That can be hugely affecting, but this show goes in a different direction; the fourth wall is much more fluid. We use it when we want it, and we scrap it when we don’t – we have our cake and eat it. That’s a Michael Boyd phrase, “can we have our cake and eat it?” [O’Briain was Michael Boyd’s Assistant Director on the RSC‘s Histories cycle]. Can we have everything with this? Can we have funny, and moving, and profound? There are moments in this play that are happy to aspire so high. I don’t want to be arrogant – we’re just doing a show – but we want to be truthful and funny and meaningful and profound all at the same time. We”ll see  on Tuesday whether it works…”

This is not a production for those who are deeply attached to the safety of the fourth wall, then, but neither is it doing anything for novelty or shock value. “The audience are involved, they’re in the room, there’s an awareness of the audience,” explains O’Briain. “But the cast are in control of when the fourth wall is up or down – it’s a Shakespearian convention in some ways. I’m in the habit, having done so much Shakespeare, of assuming that it’s all related: Shakespeare was very at ease with the audience being there. Characters reference the audience or play to them, for humour or emotional access. In Brighton [where MOTH started life as part of the fringe] it was a funny show, there are lots of good jokes. Good actors are able to be funny one minute and in the drama the next. That makes for a lovely experience – neither entirely a comedy or entirely a drama. It’s not a tragi-comedy, it’s a kind of a comedy-drama.” Here Chadwick, who has been nodding along, adds, “I would call it a comedy-drama. It’s emotionally-driven with some cerebral elements, but emotions and human elements are at the centre; it’s not an intellectual exercise, it’s very warm and human. I wanted to write something that I’d love to go and see, and that would be a really good night out. I want the audience to come out and say ‘I really enjoyed that’, having been through the whole spectrum of emotions. I like to laugh, I like to be moved, I like to leave feeling that something cathartic has happened. That’s what I’m striving to do; I’ve seen a lot of work that’s bleak and brilliant, and that’s fantastic and important, but there’s a job to entertain to be done as well. I want to bridge the two.”

O’Briain affirms this goal. “There’s an implicit challenge in the play, in looking at your own relationships. Theatre often faces you with a relationship and forces you to consider the ways in which you may or may not fall into some of the traps, manners or mistakes that they do. It’s the theatre as a mirror thing, making us look at ourselves. One of the reasons I like working in theatre is that it makes me think about my life, and I hope it makes the audience think in the same way – something stays with them about letting people in or pushing people away. Great theatre does all these things at the same time. In terms of audience experience, you can, most of the time, go to the theatre and even if show is really good, you don’t feel like you were there, and with this show I’ve tried to make the audience feel like they were there, they were part of the crisis point of these characters.”

Chadwick “loves working collaboratively with people who I trust”, and when you hear O’Briain talk about his rehearsal process, it’s easy to see why. “We’ve been mining the play. Rehearsals are like archeology, you spend a lot of time excavating. There’s never a point where you’re really finished, there could always be something else under that bit of ground… My way of working is not to set things, but constantly to be finding new colours and different things rather than trying to find a definitive version.” Chadwick explains that they have worked together since April last year, and that they did lots of work on her script before their run in Brighton, “lots of one-on-one readings and stuff. I’d seen his work and thought it was really interesting. We met, talked about theatre and found that we had very similar views. We’ve worked really closely on the text together, and when I’ve been in rehearsals and seen things I wanted to change, or things came up that the actors were struggling with, I could. What’s tended to happen is that the major redrafting has been over a period of time, between the two of us.” O’Briain joins in: “We’d read it through and play it, and every time we came to a ‘bump’, a bit that didn’t right, we’d talk it through, and then Kefi would fix it!” So there you have it, folks: they make it look so easy…

Mathematics of the Heart plays at Theatre503 until 3 March. For more information or to book tickets, visit the theatre’s website here.