Actors often go into intense physical training for a role, bulking up their muscles and getting into shape. Offhand I can’t think of any, but there must be quite a few. Probably De Niro in CAPE FEAR qualifies.
Demi Moore for G.I. Jane:

Christian Bale for The Machinist:

I remember reading an interview with Paul Schrader about ‘’Raging Bull’’ and he says that De Niro’s physical transformation was essential for the role, that it was ‘’acting, not dieting’’ and that most of the contemporary actors simply mistake one with the other…
I haven’t seen ’’Monster’’ or ‘’Private Jane’’ where Charleze Theron and Demmie Moore undergo radical physical transformations, but I guess it was worth for Charlize as she won many awards for it, I don’t know about Demmie Moore, I guess it earned her a lot of money :))
Recently I have been impressed with Michael Fassbender’s transformation in ’’Hunger’’, the character he was playing absolutely jsutified it…
I have to admit that I haven’t seen the movies with Christian Bale, but I am usually not impressed with that ’’dieting’’ Schrader is referring to, because I am aware that those millionaire Hollywood stars can afford to do it without much of a problem…
MARK IS SUSPENDED IN GAFFA
What are some of the most startling, impressive, or just plain torturous looking physical transformations you’ve seen a performer undergo for the sake of a movie role? No artificial additions, either: I’m looking for examples like Robert DeNiro’s weight gain for his part as Jake La Motta in “Raging Bull” or Sigourney Weaver’s drastic haircut in “Aliens 3” (yes, cutting one’s locks constitutes a potentially radical physical change). I know there must be quite a few Christian Bale movies of this type because he’s famous for torturing his body to get into character. Is it worth the effort and when is it going too far?