How is Hollywood pay decided?

In the middle of the film awards season one of this year's high profile stars, Natalie Portman, has revealed that in 2011, her co-star Ashton Kutcher was paid three times more than her for a film called No Strings Attached.

She said it was because of a system of 'quotes' used to negotiate pay in Hollywood. Roxane Vacca, head of Roxane Vacca Management, which represents actors including Michael Sheen says this is the reality of how pay is decided in Hollywood and it all depends on the success of your last film.