I was just about to bring up Bale. Obviously this is the opposite of what people were bringing up, but has anyone ever seen his role in "The Machinist"? The man ate like one apple and one can of tuna a day for like two straight months, and ended up weighing like 115lbs or something.
I think this points to the dedication they have to A. their craft and being immersed in their role, and B. yes the incentive of millions of dollars, where its to get jacked and ripped like Wolverine or look like Jared Leto in Dallas Buyers Club.
Also, as stated above, if you had the resources to hire the best trainer, nutritionist, and personal chef and had millions of dollars on the line as cause to follow their every word and direction, it actually might not be the most difficult things to accomplish haha