Paul Bettany believes making a good movie is about guess work.

The British actor - who has starred in films including 'A Beautiful Mind', 'The Secret Lives of Bees' and 'Iron Man' - believes there is no guaranteed way to make a successful film, and he admits many people were surprised at the success of his latest work 'Margin Call'.

He said: "I've never seen more shocked faces. Because nobody could quite believe it had come together so well.

"I thought it was bright and well-written, which is rare, and it was 17 days of filming so I just thought, 'This is a good punt.' It's just proof that you never know.

"People are always trying to work out the formula for making a movie that is well-regarded, but it's like looking at f**king tea leaves."

Paul stars alongside Demi Moore, Zachary Quinto and Kevin Spacey in the movie, and he admits it "changed" him because it showed him how professional a set could be.

He said: "This movie changed me in that it reminded me of all the reasons why I first became an actor. I can't make movies that I don't care about any more."