Into The Wild-Emile Hirsch's Body? (Not a racy question) Just wondering...?

In Into The Wild, Emile just gets skinnier and skinnier (which makes sense, considering his inability to find food, and later on, to digest food), but is it animated? Or was he just sucking in his stomach and his cheeks? (which seems unlikely) What was he doing to look so skinny? It's so mind-boggling!!! ( I dunno, I was just wondering :P )