| February 4, 2013

On the surface, it sometimes seems that Hollywood actors and actresses have the easiest jobs in the world. They get paid millions to shoot their movies in exotic locales with similarly beautiful people, and then, when their films finally debut, they bask in fame and adulation.

But many talented actors work much harder at bringing their characters to life. Some of them tirelessly research their roles and create elaborate backstories; others go method and refuse to break character for the duration of filming; and, in extreme cases, a scant few will forgo makeup, special effects and fat suits, and physically modify their outward appearances for a part.

READ: Hot Celebrity Bodies Shaped by Yoga

Here's a look at Hollywood's most daring actors, who not only dedicated themselves, but also their bodies, to their craft.

Robert De Niro

Robert De Niro had to get into fighting shape for 1980's "Raging Bull" — and then quickly lose that shape in favor of a much rounder one. In order to portray boxer Jake LaMotta in his later years (for the final segment of the film), the actor went on a four-month tour of Europe's best restaurants during a hiatus in production. De Niro came back 60 pounds heavier, reportedly alarming director Martin Scorsese with his labored breathing.

Jared Leto

Normally svelte actor Jared Leto gained 67 pounds to play Mark David Chapman,  convicted killer of John Lennon, in the 2007 film "Chapter 27." (To pack on the pounds, he would reportedly drink melted pints of Häagen-Dazs chocolate ice cream mixed with soy sauce and olive oil.) The tremendous weight gain left Leto with gout, and also made it severely difficult for him to walk around the movie set. Leto, too, has had experience trimming down for some of his roles, including Harry Goldfarb in "Requiem For a Dream" (2000) and Rayon in the upcoming "The Dallas Buyer's Club" (2013).

50 Cent

Rapper 50 Cent, also known as Curtis Jackson, dropped 54 pounds to play the part of a cancer patient in "All Things Fall Apart." In order to lose weight for the 2011 film, Jackson ran on a treadmill for three hours every day and committed to a liquid diet.

Charlize Theron

Prior to starring in the 2003 film "Monster," Charlize Theron decided to gain 30 pounds to more accurately portray real-life serial killer Aileen Wuornos. "It was about getting to a place where I felt closer to how Aileen was living," stated Theron, referring to Wuornos' lifestyle as a homeless prostitute-turned-murderer. After filming wrapped, Theron quickly shed the weight for a part in 2004's "Head in the Clouds."

Natalie Portman

The year before shooting began on 2010's "Black Swan," actress Natalie Portman began a strict exercise regimen — including ballet, swimming and cross-training — to shed 20 pounds and achieve the physique of a professional dancer. At one point during filming, director Darren Aronofsky claims he grew concerned after seeing Portman's figure. "I was like, 'Natalie, start eating.' I made sure she had a bunch of food in her trailer."

Michael Fassbender

The 2008 film "Hunger" chronicles the imprisonment of Bobby Sands, a real-life IRA prisoner who participated in an ultimately fatal hunger strike in 1981. Michael Fassbender knew he'd have to lose a lot of weight to take on the lead role, so he practiced yoga, took long walks, and sustained himself on a single can of sardines every evening. Over the course of ten weeks, he trimmed his 160-pound frame down to just 127.

Rob McElhenney

For the seventh season of "It's Always Sunny in Philadelphia," series creator and star Rob McElhenney thought it would be more realistic if his character Mac got fatter and worse-looking. With the help of a nutritionist (suggested to him by Philadelphia Phillies second baseman Chase Utley), McElhenney gained 52 pounds by consuming 5,000 calories per day.

Renée Zellweger

Not only did Renée Zellweger have to adopt an English accent for her role in 2001's "Bridget Jones's Diary," she had to gain almost 30 pounds. The actress ate pizza and doughnuts to round out her figure, but claims she felt lethargic and unhealthy. When it came time to film the 2004 sequel, "Bridget Jones: The Edge of Reason," Zellweger hired a nutritionist to help her gain the weight back in a healthier manner.

Christian Bale

Christian Bale has changed his physique almost too many times to count. For his role in 2004's "The Machinist," Bale dropped 62 pounds by starving himself over a four-month period, surviving on only black coffee, an apple, and a can of tuna each day. A little more than a month after shooting wrapped, he gained 60 pounds (thanks to breads, pastas and ice creams) in order to screen test for the role of Batman in "Batman Begins" (2005), and eventually gained another 40 before shooting started. Bale slimmed down yet again to play a prisoner of war in the 2006 film "Rescue Dawn," then quickly bulked up for "The Dark Knight" in 2008. And, in 2010, the actor would reportedly run for "hours on end" to achieve the look of a former welterweight boxer for "The Fighter" (pictured above).

Matthew McConaughey

In the upcoming film "The Dallas Buyer's Club," Matthew McConaughey plays a man who seeks out illegal alternative medicines after being diagnosed with AIDS. The nearly 6-foot tall actor lost 38 pounds for the role by severely cutting his caloric intake. When asked what he'd first like to eat after filming wrapped, McConaughey said one of his choices was a cheeseburger, which he described thusly: "I will have some 70 percent beef, 30 percent fat ground beef … I'll prepare it all and I'll make sure that it takes three hours just to prepare. I'm going to have buns with butter on both sides, toasted and grilled. I'm going to melt the cheese on the top bun, Hellman's Real Mayonnaise. I want kosher dill pickles sliced nice and thin, diced white onions, slightly grilled until they get almost hard, and some thin jalapeno slices. And then I'm just going to sit back and let the [expletive] just drop on the ground."

Anne Hathaway

Anne Hathaway was in killer shape when she signed on to star as Fantine in the 2012 film adaptation of "Les Misérables" (the actress was in the midst of filming her role as Selina Kyle, aka Catwoman, in "The Dark Knight Rises"). The role demanded her to slim down, so she lost 10 pounds before filming began — and then another 15 during a break in production. “I had to be obsessive about it — the idea was to look near death,” said Hathaway, who reportedly ate only two thin squares of dried oatmeal paste per day to achieve her emaciated look.