The movie in question is The Machinist, for which Bale starved himself down to 120 pounds. Early in the film, he flexes the visible skeleton under his skin for deliberate gross-out effect, and it works. (Bale presumably gained back all his weight to become Batman, but had he not, John Woo would need look no further for the perfect Skeletor in his just-announced He-Man movie.) The explanation is that Bale's Reznik has not slept in more than a year and is slowly wasting away as a result. He nods off for seconds at a time, but never longer. Colleagues suspect that he's on drugs, and his behavior is at times akin to that of a crystal-meth addict -- he scrubs his bathroom floor with a toothbrush, inch by inch.
One day at the factory, Reznik meets Ivan (John Sharian), a bald, beefy redneck with toes surgically grafted onto one of his hands. Ivan seems amiable enough at first, but the next day, he manages to distract Reznik enough to make him cause a major accident, severing the arm of a co-worker (Michael Ironside, for once playing the least frightening character onscreen). Turns out no one else saw Ivan or even believes he exists.
The Machinist requires a second viewing to be fully appreciated -- not because the twist is some out-of-left-field gimmick but because it adds additional depth to what you already know. But the first viewing is rough going, because it's tough to identify with Reznik. Only toward the story's end do we get any sense of what he was like before the insomnia and the massive weight loss. Anderson ultimately humanizes Reznik, but for a long time, The Machinist seems like it will collapse under the weight of a façade composed of everything a mopey teenager thinks is really cool.
The mopes get it right once in a while, though. A carnival ride on which Reznik takes a young child -- a ghost train filled with woefully inappropriate imagery -- is designed with more loving care (and more genuine scares) than the rest of the film. Screenwriter Kosar probably got the idea from his Rob Zombie CDs, but props to him and to production designer Alain Bainee (The Art of Dying).