DAVID Greig is a man who takes to a challenge in the way an out of work theatrical takes to a spotlight.

Any playwright who has adapted the likes of demanding works such as The Bacchae or Touching the Void, or indeed Local Hero, for the stage is a person who lives to defy expectations - and perhaps even his own sense of what’s possible.

Yet, the Edinburgh-born writer’s latest adaptation, Under Another Sky, suggests a challenge of unimaginable heights. How would you even consider a book about Britain’s 400-year stint as a Roman province – part archaeological study and part understanding of how our past has been remembered – could be reimagined as a play?

How could Charlotte Higgins’ book, which analyses artifacts, architecture and ‘the capacity of the sleeping earth to throw up anomalies’ possibly be transformed into 90 minutes of theatrical entertainment?

The answer, it transpires, lay in the phrase ‘The blue van.’

David Greig smiles as he rewinds on how the seemingly impossible project came to be possible. And he admits the leap from print to stage was neither obvious nor deliberate. “I really liked the book,” he enthuses. “I’m very interested in that period, but I really didn’t think of it as a play at all. But then the Lyceum Theatre had an idea whereby we would do short adaptations of a book in a week and present them to a Book Festival audience.

“So, I thought it would be fun to try to adapt the book, really just as an experiment, certainly not as a way to go on and develop it as a play. It was just an idea about how to present ideas about history.”

The adaptation went really well. But then David Grieg picked up on a tiny piece of information which set the playwright’s imagination into fifth gear. “When I met Charlotte in Edinburgh, she told me that when she researched the book she’d spent two summers in a blue camper van with Matthew, her now partner, who is a Professor of Classics at Glasgow University.”

The idea of Under a Different Sky becoming a play suddenly took hold. “I thought, ‘Ah, this could be the way of telling a story, not an adaptation of the book, but a kind of adaptation of the process of writing the book.’”

In the play, the audience is taken on two journeys, the first where two people research, uncover and debate the impact of Roman Britain, and the second being the developing romance. “Yes, it’s a sort of road movie. And at the same time, I wanted to explore this notion of an intellectual friendship, which I haven’t seen done too often in popular culture, where people find their togetherness through thought and ideas.”

He laughs; “Normally, road movies are reliant upon car chases and big adventures, but what we have here what is ostensibly a boring subject. What Charlotte has done is make a name for herself by finding out things that may be seen as boring but manages to make them sexy. On the surface, this is so light; just two people travelling around the ruins of Roman Britain. But underneath it’s a story of how we find ourselves with each other, what connects us, and how we relate to the past.”

The play is very much about imagining. Can we really connect to someone in the past? What does a ruin mean to us? Greig has enjoyed exploring those themes. And it’s a guarantee the writer of the likes of Midsummer, or the book of the West End hit Charlie and The Chocolate Factory, will ensure it’s funny.

Charlotte Higgins, as you would expect, was delighted when David Greig revealed he planned to turn her book into a play. “I’m a huge admirer of David’s plays and I realised I’ve loved so much of his work over the years, such as Yellow Moon and Solaris, and it was a massive surprise that he decided to do this.”

She laughs: “Nobody would pick up my book Under Another Sky and think this is a natural adaptation into drama. It’s quite a serious work about how Roman Britain has interfered with our ideas of national identity.”

Yet, the book is structured as a travelogue. Higgins's determined pursuit of architecture and artefacts sees her set off across fields, council estates and garden centres, and then off to the next treasure site in the blue camper van. Along the way we are treated to historical reportage, analogy and anecdote. Lots of poetic overview. And discussions such as ‘Why was Hadrian’s Wall built?’ considering it was too long to hold back those who determined to climb it.

“Yes, but I don’t appear in the first person very much. I only appear when I want to give the reader something of a handhold to suggest that my reaction to the ancient past is just as subjective and conditional on the time I live in as the people in the 18th and 19th century. (She considers thoughts from the medieval mythographer-historian Geoffrey of Monmouth to Edward Elgar and W.H. Auden.) And well, to turn it into a road trip play has been a surprise.”

Yet was there also some trepidation mixed in with the excitement with the realisation you were going to feature in a play which offers up the story of your new relationship?

“Definitely,” she says with a chuckle. “There was a slight turning over of the stomach. I knew David’s work well, I trusted him. But having read the script, it was a peculiar experience to see myself and my partner appear. Yet, at the same time David has fictionalised us in order to explore some of the ideas in the book, and it’s a device to allow us to ask questions about the deep past, about how people can visit ancient sites and have very different thoughts about them.” She smiles; “There is as much of David in the characters as there is of me and Matthew.”

Higgins digs a little deeper into the intent of the writer. “David can see himself in other people’s shoes. But as a writer of non-fiction, I think there are serious limits as to how you can imagine the thoughts of, for example, a Roman Soldier based on the Antonine Wall in the AD 140s. We’ve talked about that stuff and in a way, I can envy his blithe ability to leap, whereas I’m standing on the side-lines thinking, ‘No, if I leap, I could leap wrong.’”

But the essence of the conversations between Charlotte and Matthew is true? “There were flashes of truth,” she says, laughing. “But what’s happened is that David has essentially created a new form. He’s dramatized the journey towards writing the book, the collecting of the research, and turned it into this gorgeous, almost rom com adventure, and it’s absolutely lovely.”

She grins; “It’s a road movie, but we’re not Thelma and Louise. We’re not driving a car off a cliff. We’re still here to tell the tale.”

Has Matthew read it? “Yes, and I was a little bit worried about his reaction, although I’m not sure why. But all I could hear was gales of laughter coming up from the basement where he was reading.”

Charlotte Higgins has, for several years, been The Guardian’s Chief Culture writer, and a multi-award-winning author of serious historical works such as Greek Myths: A New Retelling,

Now, she’s a romcom star? “Well, it’s all a huge adventure,” she says, grinning. “And I have to say that Amelia (Donkor) and Keith (Macpherson) the actors who play our characters, are great, and they will be very different from the blushing, rather awkward Charlotte and Matthew who will be in the audience on opening night.”

David Grieg once declared; ‘Writing itself kills me, though. Each new play feels like it’s my last. For some reason I can’t write without the feeling that this is it. I’ve been found out.’ Yet, he still takes on the difficult challenges, such as turning a book about the impact of Roman Britain on the mind into a play? “Yes,” he laughs. “But it’s such a great story it had to be done. And at the very least the audience will be able to enjoy the best view from any theatre space in Scotland.”

Under Another Sky, Pitlochry Festival Theatre Amphitheatre, August 10 - September 23.