Meryl Streep pulled a prank on The Iron Lady director Phyllida Lloyd by leaving a voicemail for her pretending to be Margaret Thatcher, the director has revealed.
The Oscar-winning actress stars as the former British prime minister in the new film which shows Thatcher as an old woman reflecting back on her life.
Phyllida, who worked with Meryl on Mamma Mia!, revealed: "I was shopping in Selfridges when I first heard her speak as Margaret Thatcher. She sent me a voice note on my telephone, her first attempt at speaking. And I just sat down and thought I was listening to Margaret Thatcher."
Meryl gives an uncanny and moving performance as Thatcher in the film, scripted by The Hour's Abi Morgan, which Phyllida feels is about the politics of old age, and not British politics.
The director added that it was very fitting for the American actress to play the British prime minister as it paralleled Thatcher being an outsider in a male-dominated world.
Phyllida said: "Casting her was obviously the most important decision we made and I think there's something very powerful, aside from Meryl's extraordinary talent, about the outsider, the American coming in to play the woman who was the outsider.
"Both of them had to learn how to speak, prepare minutely their costumes, their hair.
"Meryl's preparation of how she spoke was a kind of parallel of Margaret Thatcher's own preparation for going in to face those rooms full of men. And I think there's something you feel from the screen of that kind of preparation."
The Iron Lady opens in cinemas on January 6 2012.