No one has picked up on what I imagined -- that Reilly just might opt to work out a bit, not to remotely approximate Brando (or even Baldwin), but to merely turn himself into more viable a leading man. That's not me putting down a chubby fella who has every right to be a zhlub, it's suggesting that a good actor prepares for a good role--extensively.
It might end up being the balance between the two extremes posted here -- that Stanley must be ripped or Stanley will end up being a puddle of pudding. I'm sure Reilly sees this opportunity for what it is, and will begin preparing for the role immediately. That includes physical and mental work.
Curiously, if this were a case of a women, we'd all quickly say "Oh, she'll go to a spa! She'll get a trainer!" We all assume women will opt for everything from breast augmentation to starvation for their art, whereas men are considered 'what you see is what you get.' I respect Reilly, and see him seizing this day in the sun that looms for him.
"I'm a comedian, but in my spare time, things bother me." Garry Shandling