Christian Bale - the actor of 'Batman Begins', took a role of a skinny character earlier in the movie 'The Machinist'. He had to put off 63 of his 180 pounds over four months, initially by running, mostly by not eating.
How could he stay so determined?
The success of his losing weight program is this - Books. He kept on reading books that distracts him from eating. It really did convince him that having hunger pangs felt good or reading a book was as nourishing as a steak. He felt worse putting the weight back on, suffering heart palpitations and anxiety attacks. What a sacrifice for the role of skinny 'Trevor Reznick' that is about a machinist haunted by a ghostly figure he sees in the factory. I admit that this movie deserves a good rating.
Sources: Nydailynews.com, Killermovies.com; Yahoo!Movies
How could he get back in shape while filming the movie 'Batman Begins'?
"Since he had lost a great deal of weight in preparation for his role in The Machinist, Christian Bale hired a personal trainer to help him gain 100 pounds in the span of only a couple months to help him physically prepare for the role of Batman/Bruce Wayne. This effectively shattered the record of 70 pounds set by Vincent D'Onofrio in 1987 for his role in Full Metal Jacket. At first he went well over the weight required (taking literally Nolan's instructions to "get as big as you can"), and when he was first introduced to the crew there was concern over whether he'd look right for the part. However, Bale worked hard to lose the excess weight, and was ready in time for filming" - Wikipedia
I'm looking forward to watching the next sequel of Batman Begins - including old rival - the Joker!