Mila Kunis' body changed due to her intensive training for 'Black Swan'.

The actress took ballet classes in preparation for the dance movie - which she stars in alongside Natalie Portman - and admits she wasn't prepared for how "physically excruciating" she would find the lessons.

She said: "A lot of things change, your body changes. Here's the thing about ballet that I never knew about. It's one of the most physically excruciating sports that I've ever been a part of, and I say sports because they train constantly, every single day.

"Your body changes. Your shoulders drop. Your chest opens up and there's a certain posture that I naturally don't have because I slouch.

"So for three months, every time I had to constantly stand up straight and the way that they hold their arms because they always move their fingers when they're dancing, that also changes and it also changes the way that they talk in real life and amongst the feet being different because of the ballet stuff. So there are a lot of different things."

Mila admits that her graceful performance in the film only came about because she had prepared so thoroughly.

She said: "It was far from effortless and sensual. It was three months of training beforehand. I was not a ballet dancer.

"So it was three months of training, seven days a week for four or five hours a day before production started and then during production it was pretty much exactly the same."