Bill Bryson on what happened when Robert Redford demanded to play him in A Walk in the Woods

That lack of integration is a crucial part of the way America sees itself and the world, and one of the many soul-deep differences between them and us. It explains why so much of America is so ugly, and why some bits of it are still spectacularly wonderful.

Bryson has strong feelings about the fragility of much of the great trail he walked. “There’s huge pressure to build along the trail because much of the land is private, and there are no restrictions like there are in Britain. This is a very, very threatened environment.”

Time and again the film pulls back from the main two characters, Redford/Bryson and his Sancho Panza, Stephen Katz (played by Nick Nolte), and sets them as dots in a vast landscape full of dramatic chunks of rock, helter-skelter streams and trees as far as the eye can see.

“The walk didn’t change my feelings about America – it changed my feelings about the world. Just how big it is. Out in the hills, where you can’t see a house. On foot, you move so slowly through the landscape. Your idea of scale changes: you do ten, sometimes as much as 15 miles in a day, and that’s a long, long way.”

It’s a revelation that comes to everyone who visits a threatened environment. You read about how much of it has been destroyed, but when you’re in the middle of it, you realise how much is left, and understand in your guts why it’s worth saving. The difference between a rainforest half-destroyed sort of person and a rainforest half-saved sort of person is that the second one has actually been there.

Adventure in America with Radio Times Travel

A walk like the Appalachian Trail is inevitably a more-than-you- bargained-for experience. It began when Bryson took a short walk from his house in New Hampshire and realised he had connected with the trail “beguilingly leading through the woods”.

He realised that he could turn left and walk more than 1,500 miles south, or turn right and walk a few hundred more miles north. For a man with a taste for travel and in need of a book idea, it was irresistible.

“It was the hardest thing I’ve ever done,” he says. “I never want to do it again. It was unlike anything else I’ve ever done and in some ways, the best thing I’ve ever done. It was that feeling of being taken out of the world, literally out of the physical world I normally inhabit. The really big thing was that there is nothing to fill your thoughts. You don’t spend time running through all the things you have to do because there’s nothing you have to do except walk.

“So I spent the early part of the walk feeling sorry for myself and feeling homesick. Then, quite miraculously, you pass through a barrier. You accept this and your whole brain empties, in a Zen-like way. And I was complexly taken by surprise. When I finished, I was extremely fit – by my standards. But also I felt as if my brain had been cleaned out in soapy water.

“Of course, within four days of my return I was back to normal. Though I still do a lot of walking. Day walks, that is. I’m never going to sleep in a tent again.”

He is currently in that uneasy state of just completing a book and wondering if it’s any good. “I... I hope I’m happy with it,” he says. The book is a belated sequel to Notes from a Small Island, called The Road to Little Dribbling, and is published next month.

Bryson is unassuming, yet at the same time offers something rather rare – a richly human combination of humour and perceptiveness. He plays the everyman with a unique talent.

Obviously Robert Redford had to play him. Nobody else would do.

Adventure in America with Radio Times Travel