You’re directing a new stage production, 946, based on Michael Morpurgo’s wartime novel for children The Amazing Story of Adolphus Tips, which tells the story of the rehearsal for the 1944 D-day landings in Devon. The action is seen through the eyes of a teenage schoolgirl whose whole village is evacuated. How did the collaboration happen?
Michael’s been coming to see Kneehigh [Theatre]’s work and I’ve been reading his books, so we had been aware of each other and looking for a project. Michael suggested something to me, and we went off for dinner and I said: “You know, it’s not really this one I want to do, it’s The Amazing Story of Adolphus Tips.” And Michael was fantastic and said: “Do that one then!” He is a great supporter of Kneehigh and quite the most open-minded, free-thinking, generous spirit I’ve ever met.

The book is based on a painful episode in history. While training for D-day, US troopships in the Channel off Slapton Sands came under fire from German E-boats: 946 people were killed (greater than the actual invasion of Normandy months later). Why don’t we know more about it?
Many people in Devon and Cornwall don’t even know about it. It was kept secret [by both the UK and US governments]. I hate to say it, but probably for good reason – to keep morale in the country high. But the cost was devastating. And when you dig deeper it was mostly due to terrible communications and bad luck; human error after human error.

The Americans were on a different radio frequency from British naval HQ on shore, so they knew nothing of the approaching German boats...
And also the ships were full of soldiers not sailors and they didn’t know how to put out their life rafts, so most of the casualties were people who managed to sink themselves with their own equipment. Even the men who escaped the water faced further trauma. Just as they landed on the beach, the British decided to practise friendly fire and didn’t pass the message on to everybody. So they literally shelled their own men.

Many of the casualties were black American soldiers?
Isn’t it interesting, the way we airbrush history? That’s been the motivator of my work over the last few years. When I did The Empress at the RSC it was really about how many black and Asian people lived in England in Victorian times. And yet you never see a black face in our TV adaptations of Dickens. It was the same with the second world war – we had thousands and thousands of black servicemen, not only the American GIs but also from the colonies.

You say every production you do has a very personal connection to your own life, from The Red Shoes (about the end of a marriage) to Brief Encounter (the battle with monogamy). What’s the hook this time?
My mum gave me the book because she read it to my niece. But I think I’m always drawn to the war years. You can’t change who you are and I think my psyche is locked around that time because of my grandparents, who I was very close to as a child. Grandad fought in the war and my gran brought up a family alone during the war, in the countryside in Dorset. I remember Worbarrow Bay, which was taken over by the military and never given back, used to open once a year and we all used to traipse down to the beach to own it again.

What’s your favourite Kneehigh production?
It’s hard; they’re all my children. But the seminal show has to be Tristan and Yseult first performed in 2003. We had a four-week run in Cornwall and thought that would be it. But the power of that story and the production and the group of people who made it – it just built and went to the National, to America, and it’s just finished a recent tour. I feel every time we make it, it gains a deeper patina of humanity and love. We made it in our early 30s and here we are in our late 40s. We had an amazing moment when we were working on the end section recently, and everyone started crying. I turned to my friend Giles and said: “What’s happened?” And he said: “We’ve all got a lot more heartbreak to pour into it.”

How do you cope when a show isn’t coming together?
Hold your nerve. I’m a great stopper of work and drinker of wine and you know things come in the middle of the night. You just have to keep your mind open and not panic. I think circling a problem is much better than tackling it head on. Just keep swimming round it, and one day something will pop out; you just have to be ready to notice it. When I flounder, I always come back to: what’s the story, who are you telling it to and why are you telling it? Being simple is the hardest thing.

You will be the new artistic director at Shakespeare’s Globe from 2016. Yet you say you’ve struggled with Shakespeare?
I’ve struggled with the language all my life, but actually that’s why I’m delighted by it. I’m not some academic who’s studied every line. I’m discovering it for the first time. I’ve got a very fresh pair of eyes on it and a skip in my step. He’s the biggest, fattest storyteller of them all so I’m stepping in those shoes, definitely. And yes I’ll still be outdoors! I’ve smiled at the gods, wherever they might be, as I’ve sat in the Globe, and thought: “I’m still breathing the air. I’m still looking at the sky.” And I love that. I was encouraged to apply for the job by close friends and I wasn’t sure, but the process doesn’t feel like an end of anything. It feels like a beginning. As an artist the only truth is that you can’t stay still. I’m in my late 40s now; it’s time for a new challenge.

What do you watch to be inspired?
I always hate this question because I feel I do very little. I spend all of my time on the M4. I love art, I love films. I’ve got two step-boys who are 13 and 16, and I love watching their relationship with technology. And gaming, I find that very inspirational. That’s my big question at the moment: why would the young generation sit down and see a story that pre-exists and is going to happen with no impact from them? So I keep thinking we have to inspire the next generation who are used to having an impact on things that they watch and experience. But you can’t worry about being cool, because what’s cool probably hasn’t been done before, so you have to be brave and surprising and keep an open mind.

946 is in the Asylum tent, The Lost Gardens of Heligan, Pentewan, St Austell, Cornwall PL26 6EN, 25 July-23 August 2015