It's not always under medical and dietary supervision. Christian Bale was in a movie where the lead was supposed to be extremely thin. His diet was a cup of coffee, an apple and sometimes a glass of whiskey per day. 

Christian Bale wanted to go down to 99lbs for it, but the film makers stopped him because he was already incredibly thin and the weight mentioned was for a shorter actor. He ended up being 120lbs.

Then he had 6 months to get ready to film Batman. The movie was The Machinist. Good movie.