Irvine explained that Spielberg's insistence on no rehearsals for his films proved to be a unique challenge for those working on War Horse.
"It was a more difficult way of working than I was used to, especially in theatre shows, you're used to coming to rehearsal before you actually get on stage, but it's a different way of working," the actor told The Huffington Post.
"What I learned very quickly is that you have to be very present and very in the moment and most of it is just reacting."
Irvine further explained: "What you see on film might be the first time that ever happened, and I don't know what the other actor's going to do because we didn't really talk things through or anything. You just have to be very present in the moment, and if you're not very present in the moment, you fall down very quickly.
"You have to be very on the ball and in a way it raises your game somewhat because there's no time for mistakes, there's not that rehearsal time where you can make those mistakes, you have to turn up that day and it has to be good because you know full well that whatever you shoot that day will probably be in the movie, and the DVD for years to come."
Irvine recently claimed that he developed trench foot while filming War Horse.
He also joked that he ate far too much food on War Horse's set.
War Horse is now playing in the US and opens on January 13, 2012 in the UK.
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Watch a trailer for War Horse below: