I'm flying high, 38,000 feet. No, not just from euphoria, but literally. After months of research, endless delays at Heathrow - after the obligatory suspected terrorist was hauled off the plane - I'm on a Boeing 777 half way across the Atlantic. I'm on my way to New York to see the toast of Broadway, British Shakespearean actor Mark Rylance, perform a play I wrote about an extraordinary peacebuilder in the Democratic Republic of Congo. Right now, Mark is riding the wave of success. The play he's starring in, Jerusalem, has just been given the Best Foreign Play award by the New York Drama Critics’ Circle and has been nominated for six Tony Awards.

My short play, The Peacebuilder, is for a one-off performance by Mark, and it tells the story of peacebuilder Henri Bura Ladyi. Henri's been called ‘Africa's Schindler’, because he risks his life on a daily basis, going into the rainforest in eastern Congo to persuade militia groups to give up their weapons and their child soldiers. As the script-writer, it's been an incredible journey to get here. Frustrating, truncated calls to Congo, always cut off at the most poignant moments, lost tapes, a trip to see Henri with hours of waiting to meet militia leaders, only for them to turn up after days of long, drawn out talks.

The situation in eastern DRC is fairly quiet right now. But it wasn't always so. Henri grew up in a culture of violence and insecurity. Nearly six million people are estimated to have died since fighting first started in 1996. A tsunami of sexual violence has swept over the east. A report published by Harvard this week states that 48 women are raped every hour.

Tens of thousands of kids have been kidnapped or enticed into armed militias, with false promises of a 'better life' and 'meat everyday'. They're made into killing machines, all the more tragic because at the age of 9 or 10, they're so eager to please - so they kill to please, usually after being drugged. It's all the more remarkble that someone who was immersed in the quagmire of war has become an agent of peace. But as Henri told me over weeks of interviews, he didn’t always see himself as a peacebuilder: years ago he wanted to be a war-maker.

I first met Henri in 2009, when I was researching stories for a BBC film. What struck me was Henri's outward calm. He comes across as deeply thoughtful. He doesn’t rush to answer - you can really see him thinking about things. And he has to be the politest person I've ever met.

Maybe this is what makes him so good at his job. He has a veneer of total unflappability, but as I learnt after hours of interviews and listening back to him, this often masks an inner fear - because he's experienced what can happen, what can go wrong at a moment’s notice. Each time he goes into the forest to meet the militia groups, he knows he may never come back. This is what I call real bravery.

As I saw first hand, Henri's negotiating skills bring results. In 2009, he brought half a dozen kids out of the forest and I met them hiding in the shadows of an intermediary's compound. They seemed feral. Wild animals with wide staring eyes, stroking their 'magic' amulets (small animal carcasses made into water bottles) for reassurance, whispering to each other in a code they had learnt as warriors.

But over the next couple of days, I saw these kids transformed. Henri sat down with them, talked to them about their old life, the life they may have even forgotten. Some of them had run away from their families – perhaps because of a misunderstanding about a lost goat, or a row over something long forgotten. He prepared them for the life they should have, as children. Even over 24 hours, you could see a difference.

Then he gave them new clothes (second-hand, bought in the market), and the external transformation was complete. It sounds superficial, but it was as if, with the peeling-off of their old clothes, they discarded the demons and horrors of the forest. Then he prepared them to go home.

The reunions were varied. Some were incredible - like an auntie who thought that 10-year-old Freddie was long dead. It was wonderful to see a child so welcomed back, with so much warmth. Others seemed strangely non-committal. Maybe that's why the kids went, in the first place. Whatever the reaction, Henri sat with each family, talking quietly but firmly about how to help these kids integrate back into normal life.

The boys all want the same thing. Even those aged 10 or 11 are desperate to get a training, so they can work and earn a living. They all had their dreams – Freddie, the youngest, wanted to learn how to make pattiserie. Thanks to Henri, when I went back this year, I saw him working at a bakery. He was barely recognisable, he looked like a different child. He'd lost the raw fragility that these boys had, when they came out of the forest. He just looked like a normal boy. The demons of Congo seemed to have receded, and he's living a different life now. The forest, and all that he saw and experienced, have been left behind – and now, hopefully he has the chance of a normal life.

For me, writing this play about Henri has been a real joy and a privilege. As I pieced events and moments together, I felt I was riding the highs and lows of Henri's extraordinary story. To have it performed by an actor of the calibre of Mark Rylance is really incredible. I just hope that I have captured enough of Henri, for Mark to bring him alive.

It's about 24 hours until the show. I’m getting nervous now. But I can’t wait.

Tickets for the show are still available and can be purchased at thepeacebuilder.eventbrite.com

Find out how the show went, when Fiona posts her next blog, next week.