It's not always just about mentally preparing for a role.
A lot of times, Hollywood actors have also needed to physically alter their appearance in drastic ways to fully become a character for a movie, and that usually includes losing or gaining a significant amount of weight.
We've already experienced a lot of this recently with Matthew McConaughey and Jared Leto in Dallas Buyers Club, but that's just the beginning.
Take a look at some more A-listers who either shed or packed on some serious pounds for a film.
Matthew McConaughey, Dallas Buyers Club: To play Ron Woodroof, a man that seeks out illegal alternative medicines after being diagnosed with AIDS, McConaughey dropped a shocking 38 pounds.
Jared Leto, Dallas Buyers Club & Chapter 27: The actor has proven that he can enormously fluctuate his weight either way. Leto lost nearly 30 pounds for his role as a transsexual with AIDS for Dallas Buyers Club, and had previously gained a whopping 67 pounds to play Mark David Chapman, the convicted killer of John Lennon, in the 2007 film Chapter 27.
Tom Hanks, Philadelphia: The legendary actor shed 26 pounds when he played Andrew Beckett, a senior associate at the largest corporate law firm in Philadelphia, who ends up fired for having AIDS.
Michael Fassbender, Hunger: The actor went from 160 pounds to 127 pounds for the 2008 film, which chronicled the imprisonment of Bobby Sands, a real-life IRA prisoner who participated in an ultimately fatal hunger strike in 1981.
Christian Bale, The Machinist & American Hustle: Similar to Leto, Bale has been on both ends of the weight spectrum. He recently packed on 43 pounds of belly fat for his role in American Hustle, but was drastically thin in The Machinist, where he dropped 62 pounds by surviving on only black coffee, an apple and a can of tuna each day.
50 Cent, All Things Fall Apart: The rapper not only wrote, produced, and financed the low-budget project himself, but he also whittled down to nearly nothing' and lost 54 pounds for his role in the 2011 drama.
Matthew Fox, Alex Cross: The actor looked unrecognizable after losing a staggering 40 pounds to play a frighteningly thin and ripped tattooed serial killer in the thriller.
Renée Zellweger, Bridget Jones' Diary: Zellweger enjoyed pizza and doughnuts to pack on 30 pounds for her role in the 2001 film.
Michelle Williams, My Week With Marilyn: Williams put on some extra weight for the biopic to accomplish the icon's famous curves and size 14 body.
Hilary Swank, Million Dollar Baby: Swank gained 19 pounds of muscle for her boxing flick. "My training was two and a half hours of boxing and approximately an hour and a half to two hours lifting weights every day, six days a week," she told Movieweb.com. "The producers asked me to gain 10 pounds of muscle. I gained 19 pounds of muscle. I started at 110 and went to 129. I would drink my egg whites because I could never eat 8 to 12 egg whites in a sitting. It's just the worst thing ever."
Charlize Theron, Monster: Prior to starring in the 2003 flick, the blond beauty decided to gain 30 pounds to more accurately portray real-life serial killer Aileen Wuornos.
Anne Hathaway, Les Misérables: Her role in the 2012 film adaptation demanded her to slim down, so the actress lost 10 pounds before filming began, and then another 15 pounds during a break in production.
Natalie Portman, Black Swan: The brunette beauty dropped 20 pounds off her already tiny frame and practiced up to 16 hours a day to get into character for her role as a ballerina in the psychological thriller. Meanwhile, her costar Mila Kunis, also spoke out about losing 20 pounds for the role and weighing only 95 pounds while filming.