In conversation with director Marianne Elliott, Stephens said: "I've never written a play like this before. Normally I'm a ferocious planner, but this time I wanted to do something completely different.
"I wanted to try and release the instinctive, the unpredictable. I knew how the characters were affecting each other, but didn't know what they were going to say next. It was daring and uncertain, and exciting."
The play sees two characters, a 70 year-old man and a 40 year-old woman, in a chance encounter that turns into six weeks of conversation, changing one another's lives as they cross from London to the US. Anne Marie-Duff and Kenneth Cranham star.
Stephens added: "I think it was perhaps the most fun I've ever had writing a play. People should be able to tell in the writing but I just had a ball, I really enjoyed myself and the sense of exploration – I should do it more often."
During the conversation, Stephens reflected on where the play came from: "There's something in our country, people are worried about science being separate from the human experience and the imaginative world or creativity. And I used to be like that.
"I used to think science was for geeks or weirdos, but not for punks. Things changed while my son Oscar grew up – his obsession and love of science was like nothing I'd ever encountered before, and I fell in love with his love. This was when I found the Heisenberg Principle."
Both Elliott and Stephens grew up catching the same bus together, a fact that was only recently discovered: "I was 14 and Marianne was 16. We used to go to school together on the same bus, though we didn't know who the other one was and we didn't talk. She was the cool sixth former."
Since then the pair have reacquainted professionally, having worked together on a number of productions including, most famously, the National Theatre, West End and international tour of A Curious Incident of the Dog in the Night Time.
Elliott commented: "What I'm drawn to in Simon's work is that there's a huge world that goes beyond the lines and script. There's a well of yearning in these characters, and there's so much emotion. You're looking at these two characters as if through a microscope, but the world still moved around them in a way that was unpredictable and exciting, and very beautiful."
Heisenberg – The Uncertainty Principle runs at Wyndham's Theatre from 9 October to 6 January 2018, with previews from 3 October.