In addition, she said that she wanted to set the video to 'Fat Bottomed Girls' by Queen but Instagram copyright wouldn't allow it - which is a shame, as it would have really put the icing on the post.
The video has already been viewed over 999,000 times with lots of followers writing their support for Hathaway in the comments.
She is yet to reveal what role she is preparing for, but she has upcoming parts in dramas The Lifeboat, about a young woman on trial for murder, and Live Fast Die Hot, about a New York socialite whose life changes when she becomes a mother.
Hathaway certainly isn't the first and definitely won't be the last actor to undergo a significant body transformation for a film role.
In 2004 Christian Bale lost 63lb for his part in the psychological thriller The Machinist. In contrast, he piled on the pounds for 2013's American Hustle.
More recently the Stranger Things actor David Harbour gained a considerable amount of muscle for the lead role in the upcoming Hellboy movie.