I read The Grapes of Wrath in that fierce span of adolescence when reading was a frenzy. I was all but drowned in the pity and anger John Steinbeck evoked for these people, fleeing Oklahoma to seek work but finding nothing save cruelty, violence, the enmity of immoral banks and businesses, and the neglect by the state of its own people in the Land of the Free. The novel was published in 1939 and delivered a shock to the English reading world.

But for years I did not read him. Earlier this year, when asked to make a film about Steinbeck for the BBC, I went back with apprehension. The peaks of one's adolescent reading can prove troughs in late middle age. Life moves on; not all books do. But 50 years later, The Grapes of Wrath seems as savage as ever, and richer for my greater awareness of what Steinbeck did with the Oklahoma dialect and with his characters. It is just as alive, with its fine anger against the banks: "The bank – the monster – has to have profit all the time. It can't wait … It'll die when the monster stops growing. It can't stay in one place."

We started filming with a small crew in Oklahoma, near the spot where the novel begins. This summer there was another drought, as there had been in the 1930s. They farm land better now, but even so, many farmers are going bust. The resonances with contemporary America were powerful: the working and middle classes have once again been holed by the big banks. Once again, the protests have started up, as Americans scan their continent for work. As in the 1930s, there is a powerful feeling that the promised land promises nothing, not even hope.

In Steinbeck's day, this was part of the American dust bowl. "Every moving thing lifted the dust into the air," he wrote in The Grapes of Wrath. "A walking man lifted a thin layer as high as his waist. An automobile boiled a cloud behind it." Archive footage of the time shows dust storms swirling across the flat lands like tornadoes.

In the novel, the Joad family are driven off their farm by the banks. They pile, all 12 of them, into a truck which takes aim for the west coast, more than 1,000 miles of desert and a mountain range away. Although Steinbeck was not a Christian, he plundered the King James Bible for stories (Cain and Abel became East of Eden) and for the pulse of his prose. The family of 12 on that truck are as the 12 tribes of Israel seeking liberation. The truck itself is an ark; there is even a man named Noah on board. It was this journey that my camera crew and I followed, often down Highway 66, "the main immigrant road … the path of people in flight, refugees from dust and the shrinking land, from the thunder of tractors and shrinking ownership". Upwards of half a million Americans migrated west in the space of two or three years in the 30s, the biggest internal migration in US history.

What happened to the Joad family was an attempt to keep them and everybody like them out of California. In effect, the state unilaterally seceded from the rest of the country, refusing entry to their fellow Americans and criminalising them. There were beatings, and the loss of civil rights. The Nation magazine reported that at a place called Salinas, near the Californian coast, "something shockingly like a concentration camp had recently been constructed … a water tower rises in solitary grandeur in the midst of the camp. Surrounding the tower is a platform splendidly adapted for observation, night illumination and marksmanship." In September 1936, a pitched battle was fought in Salinas between the forces of agribusiness (stiffened by 250 proto-fascist American Legionnaires and 2,000 local vigilantes) and workers who had been forced to accept less-than-subsistence wages, forever undercut by the desperation of other workers prepared to take any wages. They were loosely organised by communists, but mostly driven by hunger.

Undercover on the bread line

Salinas was Steinbeck's home town. It made him, and after that street battle, it made him anew. His birth house is now a museum. It is a detached building, on what was in his boyhood the upper professional class road in the town, as Victorian as you could imagine. Fine bricks and wood, good-sized and plentiful rooms, sturdy furniture. On the wall there is a Christmas photograph of Mr and Mrs Steinbeck and their children, every one of them dressed as if for church. Every one of them is reading a book. The camera receives not a single glance. The Steinbecks are engaged in things of the mind.

Steinbeck studied science at university, but from an early age declared himself to be a writer and set up an unrelenting daily routine. His intellectual fascinations were great literature and biology, especially marine biology. His whole world view began in a rock pool and swept up to a study of the stars.

He had written articles about the migrants passing through Salinas, and worked at menial jobs around California for months during his protracted university years, but The Grapes of Wrath proved radically different. It was as if he had transplanted himself into another class, and into areas of passion and politics he had only observed before. A previous novel, In Dubious Battle, was an examination of earlier labour battles, but he wrote of that book: "I wanted to achieve a kind of detached perspective. I'm non-partisan, I'm just going to report, as a journalist, what's going on." In that curiously bloodless book, the communist organisers are as manipulative as the landowners themselves. In Dubious Battle was his rock pool. He was the examining scientist.

In The Grapes of Wrath you feel (correctly I believe) that Steinbeck was a core participant. What had changed him? In my view, it was probably a man called Tom Collins. After the battle of Salinas, Steinbeck decided to go undercover for months, to research what would become The Grapes of Wrath. He contacted the headquarters of the Farm Security Administration in Washington and said he wanted to work as a migrant. They assigned him to Collins, a camp manager at Arvin in California. The two men worked in the valleys for several months in 1937. Steinbeck dedicated the book "To Tom – who lived it".

The camp Collins ran features like a utopia in the novel. We filmed there this summer, and it is deeply touching to see that Collins not only ran a rare, uncorrupt and democratic camp, but had put up a schoolhouse, a library and a meeting hall. Collins and Arvin are at the moral centre of the book; what he learned there gave Steinbeck the vision and mass of knowledge he needed to write the book. He learned how to keep battered trucks on the road, what food was possible on the poverty line. His descriptions of physical work are authentic, as are the flashes of human kindness and the constant stab of inhuman cruelty.

Steinbeck wrote furiously and said that the effort nearly destroyed him. "I'm trying to write history while it is happening, and I don't want it to be wrong." He added: "[I]t is a mean, nasty book and if I could make it nastier I would … the book has a definite job to do … I want to put a tag of shame on the greedy bastards who are responsible for this." He took his title from the Book of Revelation, via the triumphalist 1861 Battle Hymn of the Republic, reprinting it in full at the beginning of the novel.

A liar and a communist

It was the bestselling book in America in 1939. A film version starring Henry Fonda and directed by John Ford followed, itself a classic. Arthur Miller wrote of Steinbeck, "I can't think of another American writer, with the possible exception of Mark Twain, who so deeply penetrated the political life of the country." And yet Steinbeck was also called "a liar", "a communist" and "a Jew acting for Zionist-Communist interests". The book was burned in the streets; it was banned in schools and libraries, with its explicit sexuality given as the excuse. It was virulently attacked in Congress, and Steinbeck's subsequent success in Russia eroded his reputation from the cold war onwards. He bought himself a revolver for self-defence and had good reason to fear for his life. The book has sold about 14m copies and still sells steadily.

Steinbeck went on to develop his interest in natural science and to write many more books. His large attempt was to find common ground between the observable natural world and the worlds of myths and mysticism. His reputation was blasted regularly by the new metropolitan tastemakers. The New York Times poured bile over his head the day before he won the Nobel prize, in 1962 ("The Swedes have made a serious error by giving the prize to a writer whose limited talent is in his best books watered down by 10th-rate philosophising"), though there were many fine writers who rushed to defend him. Steinbeck answered his critics in his acceptance speech in Stockholm. "Literature is not a game for the cloistered elect. Literature is as old as speech. It grew out of human need for it and it has not changed except to become more needed."