‘And did you really believe you had invented me?’ Graham Greene imagines the characters from The Power and the Glory asking as he sits quietly at home, rereading his own work. The author had visited the Mexican state of Chiapas in early  1938, correcting the proofs of Brighton Rock as he travelled round. He went not in the service of fiction. He had been commissioned to write a book about the Catholic Church in Mexico. The fruits of that endeavour, The Lawless Roads, duly appeared in 1939, a year before The Power and the Glory, a novel set in southern Mexico. In 1978, William Heinemann and the Bodley Head reissued the book with a new introduction by the author. ‘The self of forty years ago’, Greene states, ‘is not the self of today, and I read my own book as a stranger would.’

To his amazement, he found almost the entire cast of The Power and the Glory waiting patiently on the pages of The Lawless Roads. The dentist Greene had called Mr Tench, who made a living selling gold fillings in the abandoned little port of Frontera, turned out to be ‘as complete in The Lawless Roads as he was in The Power and the Glory’. The men and women Greene thought he had invented had actually been ‘buried completely in my subconscious’. The things related in both books, Greene writes, ‘really happened to me – or at least to that long-dead man who bore the same names on his passport as I do’. When he conjured his plots, he reported, ‘I was only handing out alternative destinies to real people whom I had encountered on my journey.’ Yet Greene knew that by smelting what he had observed into art, he had revealed a truth that was truer that the truth. In the words of another master, V S Naipaul, born twenty-eight years after Greene, ‘Non-fiction can distort; facts can be realigned. But fiction never lies.’

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Last month I travelled through Chiapas myself on a writing assignment. I came down from the Sierra Madre, though not on a mule like Greene. Here, in its southernmost state, Mexico veers east and tapers into Guatemala. It was the dry season, and you could see the hollows