When it comes to playing a role, physicality can be just as important as how you deliver your lines and actors have gone to some extreme lengths when it comes to physically preparing for theirs. Consider this, body language (the way we move, facial expression, tone of voice, and so much more) are a huge part of the overall communication process, so even if you are committed to emotionally delivering your lines, an actor has to look the part.
Sometime that means getting really in shape. We see this with a lot with roles that are physically demanding where you have to do a lot of stuns or simply look more in shape, and it isn’t just the guys who do this. Women like Demi Moore made a complete transformation for “G.I. Jane” (down to cutting her hair), Natalie Portman trained as a ballet dancer for “Black Swan” and Hilary Swank became quite fit for “Million Dollar Baby”.
But like people in real life, not all characters are created equal and sometime that means losing an extreme amount of weight like Anne Hathaway did “Les Miserable” or gaining weight, like when a young actor is playing someone older or has that kind of physicality.
In end, it’s important to remember that while actors are dedicated to their craft, this doesn’t mean that they can’t have or shouldn’t have a healthy lifestyle. If you honestly feel like you can’t fit the physical demands of a role, don’t worry, there will be others.