The extras were a motley crew. 'First of all there was a kind of aristocracy we called 'dress extras'. White Russian refugees - ex-generals and ex-ambassadors who knew how to wear evening clothes and look truly elegant. But there was also a lot of flotsam and jetsam, some very peculiar and unsavoury characters. At 21, to be in the middle of all that and to see human nature in the raw was fascinating.
'There was a very interesting thing about the psychology of the Germans and the Americans. A famous German cameraman got a job in Hollywood. One Saturday, at the end of the day, he called his assistant, who was an American, and said, 'Tomorrow, I want you to come to my house and wash my car.' The man looked at him, laughed and walked away. In Germany, he would have said, 'Yes sir,' and done it. This was really why I went to America.'
That said, he found a military regime in operation on All Quiet on the Western Front. 'I learnt from the worm's perspective what it was like to be an extra in Hollywood. To make 600 men function as extras, the first assistant director had several assistants under him who in turn were in charge of small groups. These squad leaders were usually ex-corporals or ex-sergeants. And they would start remembering the good old days and try to get very, very military.' After six weeks Zinnemann got into an argument and was let go: 'I'm glad I got fired, otherwise I'd probably still be an extra today.'
All Quiet on the Western Front plays at the National Film Theatre on Sunday 10 October in a month-long season of WW1 movies. 'Fred Zinnemann: An Autobiography' is published by Bloomsbury at pounds 25.