"Last spring, a stroller on Clapham Common might have seen an uncommon sight: an American directing a film set in the Edwardian era. What's so uncommon about that? Lots of costume dramas are filmed here, some even by Americans. But this American was David Mamet and the film he had scripted and directed was Terence Rattigan's The Winslow Boy. Mamet, America's leading playwright, meets Rattigan, England's last craftsman of the well-made play: what is going on here?

Last week, at a private preview in New York, I found out, and it amazed me. Not because of any in-your-face, four-letter expletive-ridden rewrite Mamet had made of Rattigan's Shavian-style drama: but because he had preserved it in every detail as lovingly as if it were a Grade I listed monument in English Heritage's care. Mamet's The Winslow Boy is exactly as Rattigan's pen, 50-odd years ago, dramatised the real-life case of George Archer-Shee, the 12-year-old Osbourne naval cadet sacked in 1910 for allegedly stealing a five shilling (25p) postal order. The injustice fired the conscience of the nation, caused consternation in the Admiralty, roused Sir Edward Carson, the famous KC, in his defence and became the cause c�l�bre of its day.

Mamet's version of Rattigan's reconstruction is clearly a labour of love: but why? Why do it all over again? Anthony Asquith filmed it in 1950, with Robert Donat as Sir Robert Morton, KC, who defends the boy, Sir Cedric Hardwicke as the stoic father who ruins his own health and his children's futures in order to prove his son's innocence, and Margaret Leighton as his loyal wife.

These roles are now filled by Jeremy Northam, Nigel Hawthorne and Gemma Jones. Therein lies the first clue to Mamet's dedication. A cast like this - to which add Mamet's wife Rebecca Pidgeon and her brother Matthew Pidgeon as the elder Winslow children - has the gift of English tongues that seduces Mamet, a playwright with the finest ear for dialogue in the US. Mamet has had us all so well hooked on urban America's blustering blue-collars and snake-eyed shysters in plays like American Buffalo, Glengarry Glen Ross and screenplays of labyrinthine tricksiness like House of Games and The Spanish Prisoner that we forget what a formidable and formal wordsmith he is: just like Rattigan.

"David's writing is enormously immaculate," says Hawthorne. "Every fractured sentence has to be delivered exactly as he wrote it." The well-spoken play is the other seductive face of the well-crafted one. Then again, The Winslow Boy is a story of an abused child - abused by the Establishment - and Mamet's childhood has its own recorded history of abuse. I'll bet the prime bait for him was the suspenseful scene in Sir Robert's chambers. Northam, in white tie and tails, preparing to dine out in London society, verbally bullies young Guy Edwards, playing the accused cadet, in order to decide if he will take the case and try to sue the Crown for redress. Northam is brilliant: as magnetic as Olivier in his prime and as handsome; but steely, stern, legally intolerant of any flaw in the ex-cadet's protestation of innocence. We think, his appalled family think, it's all up: he won't touch the case. Comes one of the greatest lines Rattigan ever wrote: "I accept ... the boy is plainly innocent." Curtain. Mamet must have found that irresistible.

Duplicity, too, is part and parcel of Mamet's fascination with people and their motives. The Archer-Shee case revealed duplicity in the highest government circles. Very much like today, the government stuck to its story to save face until the cumulative power of public opinion caused it to cave in and accept that "right be done".

Mamet has opened out Rattigan's text, just a little: scenes are set in the House of Commons, in Horse Guards barracks, at a suffragists' HQ, but they are all short scenes. In no important way do they distract from the chamber drama in Gemma Jackson's evocatively designed suburban home of the Winslows, which becomes progressively cluttered with all manner of campaign propaganda for young Ronnie - newspaper headlines, cartoons, even sheet music (written specially by Mamet to the tune of O Little Town of Bethlehem) of a pastiche song sardonically chiding "the Naughty Cadet".

Mamet has stubbornly retained Rattigan's stagey anticlimax. We never see Sir Robert's great courtroom triumph: as in the play (and Asquith's film) we only hear the maid Violet, played by Sarah Flind, gabble out the verdict excitedly: it is enough. Tears come.

I only wish Rattigan were alive to see such favours as have been lavished on the old warhorse. I knew him in the Sixties, when his opening remark to me at his Eaton Square flat, where I'd been summoned after a devastatingly cruel review of his film The VIPs, was: "You know, of course, that I am dying." (Yes, I'd heard the rumour: but in fact he didn't die for another 14 years: Master Terry invented good opening lines as well as curtain ones.) At that time we talked about his fictional character "Aunt Edna", the average theatregoer, whom he had invented to embody all his own middle-class virtues and prejudices. Ultimately she became a millstone round his Jermyn Street collar, used by John Osborne to attack the Rattigan out-look as outworn, snobbish and class-ridden.

Yet after seeing the film, I reread Mamet's short, punchy book, True and False: Heresy and Common Sense for the Actor, published in 1997. He wrote: "I was fortunate to come up in the years when every performer entered show business through the stage ... This meant that my friends and I learned - or were given the chance to learn - the use of the age-old barometer of theatrical merit: the audience." Those very words might have been Rattigan's. Unlikely as it seems, he and Mamet are brothers under the skin: the proof of their kinship is The Winslow Boy.