After being knocked down and left gasping for air on the set of his new movie, George Clooney imposed a new rule for actors to follow – Don’t hit the director.

The star of Michael Clayton is directing sports comedy flick Leatherheads as well as acting in it, but Clooney didn’t realise just how gruelling and physical working on the football film would be.

He says: "I was going to make this 10 years ago and when this one finally got going, I realised that when I got on the football field that I actually had to play football.

"So I hired all the football players to be my height or below, and, believe it or not, it's not so easy to do anymore because actors are giants.

"The first kid that hit me was 21-years-old. I went running with the football down the middle (of the field) and he just thumped me, knocked me back and I couldn't catch my breath.

"I forgot about that part of it, so I made rules like, 'Don't hit the director!'"