LAST year, the Oxford University historian Robin Lane Fox, the author of a much-admired 1973 biography of Alexander the Great, found himself astride a horse, carrying a wooden lance, thundering through the desert dust with scores of mounted companions as Alexander's greatest conquest unfolded.

The setting was an uninhabited stretch of Morocco -- far from the Persian village of Gaugamela in present-day Iraq, where the historic battle occurred in 331 B.C. And the messianic warrior Mr. Fox followed wasn't the 25-year-old Alexander but the actor Colin Farrell, who plays the lead in Oliver Stone's ''Alexander,'' which has its premiere on Nov. 24.

Still, as the horses advanced and the cameras rolled, Mr. Fox felt epiphanies flow through him. After decades of researching often-incomplete texts about Alexander's time, he was now empirically testing history.

Yes, he sensed, you could charge with a lance without using stirrups. No, his body told him, you couldn't carry a shield in your other arm while riding. And what of the popular notion that Alexander guided his soldiers with battlefield commands? That, too, felt hollow in the noisy rattle of battle, with dust limiting a cavalryman's vision to the riders on either side. Later, Mr. Fox would become convinced that it was physically possible to run a man through with a lance from the back of a horse without losing the weapon. (Aim for the shoulder.)

''A fantastic experience,'' said Mr. Fox, an experienced horseman who demanded his on-screen riding appearance as a condition of serving as Mr. Stone's chief historical consultant. ''As a historian you're always trying in your mind to imagine how things might have been.''

Historical films routinely reinterpret or simply trash history. Indeed, Mr. Stone is a pariah in some quarters for his conspiratorial assertions in ''J.F.K.'' and ''Nixon.'' But the filming of the Battle of Gaugamela appears to have been a more complex experience, one in which there was often no historical consensus, forcing Mr. Stone and his consultants to extrapolate their grand movie reality around an earnest synthesis of the authentic and the plausible, with at least a small dollop of the merely cool.

By putting more than 1,000 actors and extras in classic Macedonian battle formations on the Moroccan desert, along with chariots, horses and camels, Mr. Stone set in motion scores of questions about what it meant to be a soldier two millenniums ago, ranging from massive to technical to trivial: How did Alexander trick a Persian army four times the size of his 50,000-man force into stretching its flanks until a convenient hole opened? Why didn't the lethal scythe-wheeled chariots of Darius, the Persian king, do more damage? How fast could a syntagma of soldiers (16 rows of 16 men) move while carrying 16-foot-long, steel-tipped spears known as sarissas? Did infantrymen wear socks?

Answering those questions fell largely to two opposites: Mr. Fox, 58, an academic (who is also a newspaper gardening correspondent), and Mr. Stone's longtime military-battle adviser, Dale Dye, a 60-year-old retired Marine captain who is generally skeptical about historical scholarship.

As Mr. Dye had done on many other film sets involving combat -- his credits include ''The Thin Red Line,'' ''Saving Private Ryan,'' ''Starship Troopers'' and Mr. Stone's ''Platoon'' -- he put cast members and extras, including hundreds of Moroccan military men on loan from the government, through a three-week ''boot camp'' that included sword fighting and practicing with other archaic weapons, as well as nighttime history lectures. The battle they sought to recreate had a mythic quality: Alexander's army had twice before clashed with Darius and won. The third victory spelled the defeat of the Persian Empire, gaining Alexander control of Asia en route to his conquest of 90 percent of the known world -- all before his death at 32.

Mr. Dye, who would direct some of the scenes in the 11-minute-long Battle of Gaugamela, was unabashed in suggesting to Mr. Fox that insights gained from putting a mock army into the field would be more credible than the descriptions of historians. Part of the problem -- for scholars as well as directors -- is that none of the 20 accounts of Alexander's life believed to have been written by his contemporaries have survived. The next closest accounts were written four centuries or more after Alexander's death, according to Mr. Fox, who describes his own Alexander biography as more a ''search'' for the general than a narrative of his life.

''I don't give a [expletive] about what the ancient sources say,'' Mr. Dye said in an interview in his suburban Los Angeles home, which doubles as a personal military museum and library. ''I went into this in the sole belief that the infantrymen had the same heart and soul in his job as the guy walking around Baghdad now.''

Mr. Fox had been retained by Mr. Stone in 2002 after a meeting in which the director, long enamored by the idea of making an Alexander movie, peppered the professor with questions. Mr. Stone, a man famously drawn to audaciousness, agreed to Mr. Fox's request that he be cast in the lead group of Alexander's cavalry. But the director balked at Mr. Fox's other demand: a screen credit that began ''And introducing . . . .''