Big movies are notorious for trampling on history; I have just given the year’s biggest movie the chance of trampling on a historian. In November, Oliver Stone’s film about Alexander the Great will burst on the world. I have been the film’s historical adviser and in September last year I galloped on my stallion across the Moroccan desert at the head of Oliver’s cavalry charge. We were filming the battle of Gaugamela, Alexander’s greatest victory over the Persians.

Both advising and acting roles came as a result of my book about Alexander and my lifelong study of him. Charging across the desert gave me a unique opportunity for some first-hand historical research. Can we really understand the horse-bound charges which were essential to Alexander’s famous victories if we have never tried to carry one out? It was also a fantasy and spectacularly good fun.

Alexander’s appeal lies in his youth, his feat of overthrowing an ancient empire and the mystery of aims and ideals which were never finally expressed before his death, aged 32. He was the most powerful man in his world at an age when most of us are still being sat on by our elders. He had a strong sense of his close relationship to the gods, encouraging the idea that he was the begotten son of Zeus.

In my view, he set out to reach the eastern edge of the inhabited world. Like his great tutor, Aristotle, he had seriously underestimated its extent. Tutorials back in Greek Macedonia had persuaded him that the world ran out in northwest India. His men refused to go on, but he returned to visit a supposed southern edge of the world at the mouth of the River Indus and probably to aim for a western edge beyond the Strait of Gibraltar. If he had lived, we would have been spared the ghastliness of the next global power, the Romans. The late André Malraux, that beacon of French educated culture, once told me that he admired young Alexander because at least he had the courage to die of his vices.

Stone is not the first director to be attracted to Alexander, or the first to come to me for help. Back in 1974 I found myself in London, at the Ritz, discussing plans for an Alexander movie with Gregory Peck, dressed in one of those famous white suits. He fancied himself as Alexander’s father, Philip, the man who knew how it had all begun. Twentieth Century Fox were willing to finance it, but sadly the great man died first and the torch passed to Time Life films instead.

In autumn 1977 they embarked on their script for a major Alexander series to be broadcast as “docudrama” round the world with a budget of tens of millions of dollars. In their wisdom, they chose a director, unknown in Europe, who was most famous for a film on the prisons of the American South. Our meeting in Oxford was not a great success. It was not just that the Randolph Hotel served him with green-coloured potato chips; it was that his main interests were the drugs supposedly taken by Alexander and the great man’s meeting with the High Priest of the Jews. In fact, there is not a shred of evidence that Alexander took any recreational drug, except quantities of wine. His meeting with the High Priest in Jerusalem is a pure fiction, invented about 200 years after his death.

After spending several million dollars, Time Life scaled down the project and turned it into a superficially scripted, cut-price alternative.

Three years later, just as a tutorial on early Sparta was coming to an inconclusive close in my Oxford college rooms, Steven Spielberg’s producers rang up to tell me with excitement how Alexander had “dreamt his way from the farm” to conquer the world by the age of 25.

“Steven really gets this youth thing in history,” they told me, “and he wants you to do a treatment of the childhood theme.” I took it on only as an escape route from tutorials, but before I could finish Steven struck first and sent me a telegram: “Have decided to get out of youth. Alexander is off. Steven.”

By 2001 three major projects were said to be in the air, but I was half relieved that none of them had given me a workout again. The huge television company HBO was rumoured to be budgeting up to $200 million for a series on Alexander, directed by Mel Gibson, who would play King Philip himself and preside over a script which was believed to be full of sodomy and filthy language. Instead, he filmed Jesus on the Cross with violence and in Aramaic. The elderly Dino Di Laurentiis was talking expansively about his plans for the big movie, casting the effete Leonardo DiCaprio as Alexander. The press were full of him, with only a few allusions to the parallel plans of the controversial Stone.

Two years later, it is Stone who has won and has closed the lid on an extraordinary 16 weeks’ filming. The mood of movies starts from the top and is either hellish or heavenly. I have talked to all the participants and been one of them, and I have to say that for all of us, Alexander has tended to the heavenly end of the scale. It still has to be cut and who knows what the public taste will be after the release of Wolfgang Petersen’s film about Troy? But I have seen the uncut dailies and I promise you, you are all in for a memorable treat.

When Stone invited me to London two years ago to discuss Alexander with him, perhaps I should have asked for millions of dollars and a film credit for my book. No doubt he would have found somebody else to advise him among the dozens of more prudent historians who also engage with this subject around the world. Before our meeting, however, I had arranged my priorities in case the relationship went well. I decided to ask for two rewards: a place in the first 15 of every major cavalry charge to be filmed in Alexander’s company and the words “and introducing” in front of my name in the credits.

Even Stone was taken aback by this request. He pointed out that “and introducing” would be impossible because there is a professional hierarchy in such matters. My request to ride in the cavalry charge caused him consternation too, until I assured him that I have ridden for 45 years and risked every bone, still unbroken, in my body in the yearly pursuit of English foxes. There would be health and safety problems, he hardly needed to tell me, but, “OK, I’ll tell them to do it, if I possibly can . . . we’ll have a rebel on horseback . . . you’re mad; you’re a cross between Peter Sellers and Ian Fleming.”