A ROMANCE based on quantum mechanics and an examination of friendship and free will might sound like a taxing night out – but Nick Payne’s Constellations has won rave reviews from the West End to Broadway.

The two-hander is currently on tour with Louise Brealey (Molly in TV’s Sherlock) and Joe Armstrong (the villain in Happy Valley) taking centre stage.

“It’s actually incredibly simple,” says Cambridge graduate Louise. “When you describe it, it sounds complex but it has a beautiful simplicity.

“It’s a love story, but we take you through different versions which may or may not have happened. In physics terms it would be a ‘multiverse’, but what it really means is that there are several moments in the relationship where things might take a different path, and we visit them.”

She stresses that you don’t need an understanding of physics to follow the play.

“Luckily, even though I’m playing a quantum cosmologist, you don’t have to understand quantum theory to know what’s going on,” she laughs. “My maths is rudimentary at best.

“I’ve been reading a few books about it and found it interesting but I’m not about to start giving a lecture in quantum mechanics – or even answer questions on it even in any after-show Q&As!”

Louise, who has graced the stage everywhere from the Bristol Old Vic to the Royal Court , says appearing in a two-hander is a world away from being part of an ensemble cast.

Having already appeared in Dennis Kelly’s After The End in Russia and off-Broadway, she says: “It’s got pluses and minuses – on the plus side you build a relationship because it’s just the two of you and you look after each other. On the downside, if you don’t get on you’re in trouble.

“Luckily, Joe (Armstrong) and I get on very well. I didn’t know him before this but even at the audition I could tell he was an amazing actor. I thought ‘I’ll have to up my game here’.”

She’s had plenty of experience in raising her game, having appeared alongside Benedict Cumberbatch and Martin Freeman in TV hit Sherlock, playing the flustered scientist Molly.

“They’re both such great actors,” she says. “I really had to take a step up or I wouldn’t have made it. They’re both wonderful in different ways – whenever I came away from a scene with either of them I had something to lock away in the memory bank. Martin is just so precise and the thing that really blew me away was how he handled Sherlock’s fall from the roof at the end of the second series. He just said one word – ‘Sherlock’ – but it conveyed so much, it was flawless.

“And Benedict just helped us so much. He has a lot of freedom to try stuff on the show but he’s not afraid to make big, bold choices. That’s really inspiring to be around.”

Louise, whose first professional job was playing Nurse Roxy Bird in Casualty, is now used to being recognised in public, especially after Sherlock.

“It’s about right because people are so nice,” she says. “Some young fans of the show will come up and ask for a selfie or an autograph but most people just do a kind of double take or give a nice smile…actually, they could just be smiling and I’m assuming they recognise me from Sherlock!”

But, despite TV stardom, she admits her first love has always been the stage and reveals she didn’t even want to audition for Casualty at first.

“I just love the stage,” she says. “There’s something about having an audience with you. It’s a collaborative experience, you’re creating something together – they’re suspending their disbelief and you’re playing the game with them.

“It’s an amazing job, a wonderful feeling. When you hit it, it’s just great, a proper buzz.”

Louise Brealey and Joe Armstrong appear in Constellations at the New Victoria Theatre, Woking, from Thursday (May 14) until Saturday, May 16.