‘It’s not unusual to find the actors playing the lovers actually fall in love’

“What is this feeling called love?” Dreamers have long asked the question, with no real intention of finding an answer, but eventually the scientists replied. A surge of testosterone, a flood of oestrogen. A neurotransmitter electricity storm in the brain. A shot of adrenaline, a dash of dopamine, a twist of serotonin. When it was observed that particular doses of wine or chocolate could arouse similar sensations, even poets might have come to doubt their own emotions. Is this the real thing, or is that just the phenethylamine talking?

The playwright Lucy Prebble has rarely taken things on face value. Her major breakthrough was Enron from 2009, which visited the Dublin Theatre Festival in 2010. A part-musical exploration of the ignominious collapse of that financial giant, it also served as a spry atomisation of the global financial crisis.

For her follow up, she turned her investigations inwards, into an exploration of love, depression and clinical drug trials; in short, why we feel what we feel. That play, The Effect, which opens this week in a new production by Rough Magic, puts two people under constant observation. One is a clear-eyed realist (and mildly depressive) Connie, the other is a impetuously optimistic (borderline manic) Tristan, who both submit to a drug trial for "agent RLU37", a kind of mood enhancer, and end up falling passionately, even torturously for each other. Is it love, they wonder, or is it the drug? Since the play debuted, with Billie Piper in the role of Connie, Prebble's observations didn't stop there.

“It’s not unusual – internationally in particular – to find that the two young actors playing the lovers actually fall in love during the production of the show,” she says, at home in London. “It’s a bizarre thing. In the fictional clinical trial these people are in a situation where they have an unusual amount of intimacy, not much else to do, and are going through quite a high-risk exciting event. And those are three really strong ingredients for making people fall in love. When you put on a play, you actually do very similar things.”

In other hands, this might sound like a marketing ploy. But Prebble's conversation is engaging and inquisitive, as alert to the suggestibility of human feeling in life or in the theatre as she is towards the desire to understand cause and effect. It can sound like a writer's prerogative, to understand everything. "Is there no mystery for you?" asks an exasperated Tristan, when Connie, an eternal sceptic, even clinically lays bare the psychology of laughter. In Enron, The Effect, or even her television show with Piper, Secret Diary of a Call Girl, there is likewise a sense that Prebble is a theatrical mystery solver, a natural myth buster.

“I’m very drawn to the elephant in the room,” she admits. “I’m attracted to the subject matter that feels pervasive and present, but that nobody is talking about. I suppose it’s to do with the revealing of a mystery. It’s to depict an illusion, but also to shatter that illusion.” She is currently working on a play about “the golden age of magic”, which follows the marriage between a magician and his assistant as conjuring gravitates from live entertainment to recorded forms, which she refers to as “the death of liveness”.

The illusion of theatre, though, seems more robust. Prebble, who also writes for video games (a long passion), will only write for one medium when the idea cannot be adequately expressed by any other. "Why the theatre works quite well with the theme of The Effect is that it is so entirely based on a fundamental illusion that everybody is agreeing to take part in – the audience, the actors and the crew. It's a mass agreed-upon illusion. With The Effect, the subject matter is love, but also, on a minor key, depression as well. The idea that we have all decided as a culture, to agree that depression is a diagnosable illness now. That's a trope of our time. It is worth examining. And that is what the play tries to do."

The Effect took root in Prebble's mind for two reasons. The first was cerebral, to exploit the theatrical potential of a clinical trial. "You're putting something in somebody's body and affecting their mind as well, then watching them to see what it makes them do. That's basically the experience of watching theatre." The second reason was more personal. "I was going through relationship things and thinking about how real love is, how difficult it is to make it last." Poetry and songs celebrate the initial surges of passion, she points out, but the culture reveres long term romance. "We celebrate both massively," she says, "but they don't overlap that well, you know?"

In Tristan and Connie, the lovers are drawn to each other, divided by disapproving authority figures and then fatefully reunited. Prebble invokes mythic forbears and admits that the names allude to Tristan and Iseult, the archetypal star-crossed romantics, but Prebble's names carry echoes of emotional states too: Tristesse and Contentment. Happiness and sadness are drawn to one another, opposites attract, and it's tempting to read The Effect on some level as a reconciliation of personality traits.

Prebble, who recognises, as the play does, that realists tend towards mild depression (happiness in life and love, psychologists agree, requires a degree of self-delusion), tends to identify with Connie’s position, “who prizes reality”. (When, in a recent interview with the playwright Simon Stephens, she repeated Connie’s glum observation that “you and everyone you love is definitely going to die”, it’s hard not to see her as the playwright’s alter ego.) But lately Prebble has come to see the wisdom in Tristan’s position, the glass half-full perspective. What does it matter, he says, if we are led by chemicals in our system? Can’t we accept things for the way they are? “I think there is something quite profound in that,” Prebble says. “There is wisdom in saying, I would rather be happy. It’s quite a mature response and I didn’t used to think it was.”

On another level the play is an exploration of the age-old philosophical struggle between free will and determinism. When Dr James points out to Lorna that knowing you have a bias doesn’t mean you can do anything to affect that bias, she adds, “It’s one of life’s tragedies.” (Prebble also intended it as a mordant comment on psychotherapy; knowing your bad patterns doesn’t always make you less inclined to repeat them.) Greek tragedy, from which Prebble borrowed to depict the fall of Enron, is a similarly fatalistic form – even when the oracle or the soothsayer advise you against a course of action, you can’t help doing it.

On stage, at least, Prebble allows for more room to manoeuvre. Her text begins, with sardonic wit, by specificing her characters’ precise heights and weights, less a tyrannical casting demand than a joke about the invasiveness of clinical observation. But her play leaves room for productions “to mould the text around themselves”.

“My experience is that there is no point in doing a theatre piece if you don’t allow quite a bit of collaboration,” she says. It comes back to free will and determinism, she adds, or whether we rush to act on a feeling or stand still to question it. “If free will is an illusion – which essentially it is – it is better to live as if it is not.” Even the room she leaves in her plays is a model of that, another experiment under observation. “A complete freedom of artistic choices is totally false. It’s the illusion of choice, within my authorial determinism.”

Being aware of that artifice, of course, doesn’t lessen its effect. In love, as in theatre, some illusions are worth holding on to.

The Effect by Rough Magic Theatre Company runs at Project Arts Centre, Dublin, from March 23rd to April 1st