Captain Dye of the US Marine Corps stubs out another Marlboro and pauses to consider the question of what he did when he retired after 22 years of military service.

"The mafia wasn't hiring," he says dryly, "so I went to LA."

If you saw Dale Dye on the set of a war movie, you'd laugh and say to yourself, "Boy, that one's straight out of Central Casting!" He is tall and angular, with craggy features and bristling silver-white hair (moustache to match). To describe his voice as gravelly is hilarious understatement; it's the sort of voice that can only be achieved by years of dedicated smoking and whiskey-drinking.

By the way, no one calls him "Dale". It's "Capt Dye", at all times. Even though he is on the set of a movie.

"I came out to LA in 1985," he drawls, "full of naivety. I became convinced I could apply everything I'd learned in the US Marine Corps to the training of actors and the making of movies."

He read somewhere that Oliver Stone was making Platoon and - "like a marine on a mission" - found a way of buttonholing him.

"Oliver and I sort of hit it off," he says. "We're yin and yang: the mad-dog conservative and the even madder-dog liberal. But we liked each other; we're close friends."

As Vietnam vets, they both had a commitment to render the experience of fighting and war in as truthful and realistic manner as possible. Platoon went on to win four Academy Awards, and in the process sealed Stone's reputation as a filmmaker and ensured Capt Dye has been in work ever since as a military consultant to the film industry. After further collaboration with Stone on Born on the Fourth of July, Dye went on to work on Saving Private Ryan, with Steven Spielberg and Tom Hanks. Which explains why he is sitting in an office at Hatfield Aerodrome, for his most recent mission has been to put the cast of the latest Spielberg/Hanks second world war epic, Band of Brothers, through their paces.

The $120m mini-series has been the subject of controversy ever since filming started in Hertfordshire last year. First we had the re-run of the argument that accused Spielberg and Hanks of historical revisionism à la Saving Private Ryan - inflating American heroism and ignoring the wartime contribution of the Brits and the rest of the Allies. Then there were those who looked askance at Steven Spielberg's honorary knighthood, and wondered whether it had anything to do with the fact that Euan Blair got a job as a runner on set.

The fire was further stoked when it was reported that the BBC had spent £15m as co-producer - a sum later corrected variously to £10m and £7m. Finally, in August, BBC1's controller Lorraine Heggessey seemed to call into question once again whether this had been money well spent when she bumped the series off her schedule and into BBC2's, saying that she did not believe the series had "a broad enough appeal to run on BBC1 at peak-time". The irony now, of course, is that with America poised to go to war once more, the appeal of Band of Brothers could probably hardly be broader.

The reason Band of Brothers was filmed predominantly at the former British Aerospace factory in Hatfield is because, as with Saving Private Ryan before it, the UK government granted the production a hefty tax break. With backing from Home Box Office, the 10-part series is adapted from a non-fiction bestseller of the same name by Stephen Ambrose. Based on soldiers' letters and diaries and hours of interviews with surviving veterans, it tells the story of Easy Company, 506th Regiment of the US Army's 101st Airborne Division - from training in Georgia in 1942, parachuting into France on D-Day, to the capture of Hitler's Eagle's Nest at Berchtesgaden.

By its very nature, Band of Brothers is an ensemble piece: in the production office photos of the principals cover an entire wall. It's hard to pick out any stars, but David Schwimmer plays against type as a sadistic training officer. Donnie Wahlberg is easy to pick out for his likeness to brother Mark. The closest thing the mini-series has to a star, or potential one, is the man who plays the company's commanding officer Dick Winters, and he happens to be a Brit.

Going from Capt Dye to Damian Lewis involves a rather dramatic shift of culture: from the Marine Corps to the Old Etonian Football Club, and from Hollywood studio to British stage. It is the job of Lewis to impersonate a soldier perfectly; and it is the job of Dye to make that possible.

Lewis has some experience of acting in a uniform: his break came in 1999 with a part in Peter Kosminsky's Bafta-winning drama for the BBC about UN peacekeepers in Bosnia, Warriors. But since Dye's theory is that "there's no way we could get an accurate portrayal unless the actors had walked a mile in the infantryman's shoes", the entire company was sent to bootcamp a fortnight before shooting started in April 2000. "Under the watchful eye of Captain Dye," says Lewis, in a somewhat rueful tone.

That's shooting as in film, not people. But if you took a tour round the aerodrome late last year, you could be forgiven for mistaking where and when you were. The sheer hardware alone is impressive: dozens of tanks, jeeps, trucks and ambulances fill the colossal hangars. Out on the former airfield an entire Belgian village has been erected; inside another hangar several hundred pine trees have been trucked in to recreate the forest of Bastogne. There is a shoot going on in there, but the snow on the trees and on the ground does not melt under the lights. It is made of paper supplied by a company named Snow Business.

Another hangar houses the wardrobe department, kitting out the actors and, on occasion, thousands of extras with costumes whose period detail - even down to the number of stitches used in the replica 101st Airborne badges - borders on the obsessive. A whole sub-department is devoted to "distressing" the soldiers' uniforms, so that it looks as though they've been living in dugouts for weeks. "Rauchen Verboten Bitte" requests a Gothic-script sign on the wall with a touch of Thespian camp. But another notice reminds actors and extras to remove ammo from pockets prior to dry-cleaning: "These rounds will explode!"

Behind security fencing are two metal containers housing the rifles, pistols and machine guns used by the actors. These are not replicas, but the real thing, which had to be acquired from arms dealers for Saving Private Ryan. The armoury belongs to a jovial Cornishman named Simon Armitage, another war movie veteran. (He made the broadsword Mel Gibson used in Braveheart.) His lieutenant is an ex-soldier named John Nixon who wears a bandana. His pride and joy is a four-barrel 0.5-calibre anti-aircraft gun capable of firing 3,000 rounds a minute, known by infantrymen as the "meatchopper".

"A nice piece of kit", he remarks.

The Band of Brothers production is on such a scale that in an ad hoc, inadvertent way it gives one a powerful sense of what really was accomplished during the D-Day invasion - the extraordinary logistical effort of moving men and matériel in vast quantities. Another, smaller hangar contains a more macabre echo of wartime: a room the size of a sports hall is filled with prone figures, pseudo-corpses used in battlefield scenes and the liberation of Belsen. A scene from hell, eerie behind closed doors, it looks like a massive installation by the Chapman brothers.

Several actors have become close to the real-life soldiers they are playing, who have been employed as consultants. I catch Wahlberg between takes in the forest, his skin made-up blue and nose red to simulate the effects of a bitter Belgian winter. He plays a guy named Lipton.

"The real Lip and I are pretty close," he says. "He definitely sized me up and put me through it a bit at first. He asked how old I was. I said, 'Er, 36.' He was 22 when he served."

One of Dye's assistants present on the set is a former British Marine corporal named Billy Budd. He was christened Adrian, but his sergeant called him Billy and eventually he changed his name by deed poll. He is sorry to see filming nearing an end.

"I'll be sad to see this go," he says with feeling, "because it's like when you've been drafted out of your platoon. It's that feeling with the guys: they're actors, but for eight months with us, they've been soldiers."

 Band of Brothers, Friday 5 Oct, 8.30pm, BBC2; repeated on Wednesday 10 Oct, 10.35pm, BBC1