I believe Natalie Portman lost something like 20 pounds to play the lead role in this dark movie. You can really see the musculature of her body especially in her arms and back in some of the close ups she gets.
This must have been incredibly difficult challenge for her, as she was already pretty tiny!
Now, I am not advocating this much of a loss in body weight for someone so small, especially not in a short amount of time. However I am pointing out that even if you are slender and fit, with some hard work you can really get into a shrink wrapped body and look damn good!
Go check out the movie!