How so?
The uniforms were pressed, the guys were clean, and they reacted in such a heroic fashion to life in extremis. No matter what Sir Laurence Olivier had to say, it isn’t just acting when something is so out of your frame of reference. You have to have an idea of what the truth is. Nobody did.

How did you get involved?
Oliver Stone was doing a movie called Platoon, based on his own experiences in Vietnam. I called him after I had clandestinely obtained his phone number. He liked that backdoor approach, we met, and he ended up letting me take his cast of 30 actors into the mountains of the Philippines.

What awaited them there?
I made them dig fighting holes, where they lived for three weeks. I fed them only rations, twice a day. If they p***** me off, they only got one meal. They carried 70lb (32kg) packs and weapons at all times, and did long patrols with only two hours’ sleep. My troops were constantly hitting them with rifle fire and explosives.

How did they respond?
They began to understand that the sun didn’t revolve around their a****. I never allowed the word “movie” to be said or for them to be called by anything other than their cast names. Within a week they had lost all sense of reality. So when we began to shoot, they weren’t acting, they were responding to real things they remembered from their training. Vietnam veterans said: “Yep, that’s the way it was.” That’s the highest accolade I could have got.

Who were your star pupils?
One of the guys who impressed me most was Tom Berenger. The script called on him to be the sergeant and he really stepped up as a leader. I’ve also worked with Tommy Lee Jones, and there’s very little Hollywood about him.

You’ve since embarked on an acting career of your own. Are there any similarities between film-making and the military?
They both have to be self- supporting and do any number of odd jobs in distant locations. The military is probably the best school in the world for an actor. You have to sublimate your own personality and become something else. I’m good at that.

Platoon: The Ultimate Edition is out to buy on DVD on March 6. Interview by Ed Potton