Ever since Robert De Niro gained 60 pounds to play washed-up boxing champ Jake LaMotta in the 1980 film "Raging Bull," actors have been using extreme weight loss and gain to help transform themselves into their on-screen characters.
Colin Farrell appears to be the latest. A recent photo snapped in Spain shows an emaciated Farrell with hollowed cheeks, sunken chest and stick-thin legs. It is believed that he shed the weight for his latest role as a Bosnia War photographer in a new film called "Triage," which is currently shooting in Alicante, Spain.
While these days it seems everyone in Hollywood is trying to pull a De Niro, shape-shifting actors can earn the kudos and respect of their peers. George Clooney in "Syriana" and Charlize Theron in "Monster" both won Academy Awards after they packed on the pounds. Renee Zellweger and Tom Hanks were both nominated after they gained and lost weight, respectively.
Some doctors worry that such dramatic transformations can give audiences the wrong impression that they can fatten up and drop weight in a flash.
"It's not as easy as it looks," Madelyn Fernstrom, associate professor and director of the Weight Management Center at the University of Pittsburgh Medical Center, told ABC News.com. "I counsel patients all the time that these people have medical monitoring, special diets and exercise regimens."
Dr. David Katz, an expert in nutrition and public health at Yale University Medical Center and an ABC News contributor, agreed that actors can lose and gain weight carefully and responsibly by undertaking a medical screening first. He warned that older actors could encounter some health problems and child actors should never attempt such weight swings.
And, he added, it's always easier on the body to go down, rather than than up, in weight. "Our bodies are better adapted to dealing with starvation than obesity."
That's good news for Farrell, who joins a group of A-list actors who literally lose themselves in a role, becoming nearly unrecognizable on screen.
If Farrell's newly gaunt look turns out to be for a film, it won't be the first time he's undergone a massive body transformation for a role.
To play Alexander the Great in Oliver Stone's 2004 epic flop, "Alexander," the Irish actor worked out for months to bulk up. He kept the same buff physique to play the cinematic version of Sonny Crockett in "Miami Vice" two years later.
Tom Hanks was apparently determined to make his role as a businessman stranded alone on an island in "Castaway" so convincing that he also took a break during the shooting of the film.
During the break, he shed 55 pounds and didn't shave or cut his hair for weeks. He told a British interviewer that the diet and exercise regimen was one of the toughest things he's ever had to do.
"The hardest thing was the time," he said. "I wish I could have just taken a pill and lost all the weight but the reality was that I had to start in October knowing that we were going to go back in February. "The idea of looking at four months of constant vigilance as far as what I ate, as well as two hours a day in the gym doing nothing but a monotonous kind of work-out – that was formidable. You have to power yourself through it almost by some sort of meditation trickery. It's not glamorous."
Zellweger gained weight not once, but twice, to play the title role in "Bridget Jones's Diary" and the sequel, and has referred to her preparation for the part as "boringly technical."
According to the Daily Mail, Zellweger, who is a slim size six normally, added enough pounds to her frame to play an average size-14 woman. At the time she was filming the second installment, "Bridget Jones: The Edge of Reason," she told About.com that she felt the role called for the extra weight.
"It had to be authentic to me," she said. "It had to be. And if you're not going to become the character and be the character, then I don't really see the point in undertaking the experience. I wanted to have that experience and people were suggesting to me, 'Oh, it might not be necessary.' Or, 'You shouldn't do as much as you did last time because it's probably not healthy.' For me, then it would render the experience pointless from a creative perspective. I wanted to revisit this character in every respect."
But whether she will revisit the character a third time seems less certain – especially if it means gaining and losing weight a third time.
"Can I just tell you my body is whacked by the time we finish one of those," she told a reporter at the Daily Mail last December. "I had a panic attack with all the specialists talking about how bad this is for you, long term, putting on that much weight in short periods of time, and they're all saying, 'You must stop this now or you're going to die'."
For the film "Chapter 27," in which Jared Leto plays John Lennon assassin Mark David Chapman, the normally thin young actor gained 62 pounds. Packing on the pounds wasn't easy.
Leto, who was a vegetarian at the time, would down a large Dominoes pizza every night for dinner, he told an interviewer on Sirius satellite radio. "It actually wasn't any fun. Some people think, 'Oh it was enjoyable,' but I was force feeding myself to such an extent that I would throw up in the back of my throat every day. My body was falling apart, and it was just really bad."
He also got treated differently. "There [were] some funny situations where a very powerful person ran into me, and they looked at me, and they were like, 'Oh, you've just grown up, haven't you?'" he said.
When filming wrapped, Leto couldn't wait to take off the weight. "Eight days after I was done with the film, I fasted for 10 days, and I got right back on the road with [rock band] 30 Seconds to Mars."
He told about.com that gaining weight was harder on his body than losing was. "I felt absolutely fine in losing the weight," he said. "It was putting the weight on too fast that I felt that I was pushing things too far."
To play the super-svelte Deena Jones -- modeled on naturally thin Diana Ross – for the film "Dreamgirls," Knowles lost 20 pounds, an idea that, she told reporters at the Cannes Film Festival premiere, was her own.
"The director [Bill Condon] actually kept telling me to 'eat, eat, eat' for continuity, but I really wanted to go all the way," Knowles said. "I was inspired by Tom Hanks. He did it all the way [for "Castaway"], so I said, I can do it all the way!"
"At the beginning [of filming], I was my normal weight," she said. "I actually gained a little bit, but then I lost 20 pounds when I went from young Deena to old Deena. I wanted it to be something more than just make-up and hair. I wanted to see the change."
For his role as former CIA agent Robert Baer in the film "Syriana," George Clooney added 35 pounds to his pretty boy persona and rendered himself nearly unrecognizable.
"I'm sort of happy at the idea that people, when they first see me in the movie or when they first see the poster, don't even know it's me," he told reporters at the time the film came out. "To me, that makes me proud."
But the extra pounds also weighed him down and contributed to the tear in his dura, creating a cranial spinal fluid leak. That threw off his basketball game and his love for motorcycling. "It was depressing," he said.
It paid off in the end. Clooney took home an Oscar for best supporting actor.
Clooney's good buddy Matt Damon lost 40 pounds to play a drug-addicted Gulf War Veteran in "Courage Under Fire." The role nearly ruined his health.
In an interview on "Inside the Actor's Studio," Damon said his weight-loss regimen consisted of a diet of chicken breast, egg whites and one plain baked potato per day and a large amount of coffee and cigarettes. He also ran six and half miles in the morning and at night.
After the film wrapped, he said on the Oprah Winfrey Show, he had to be under medical supervision for several months for damage done to his adrenal glands.
Robert De Niro
At the beginning of "Raging Bull," De Niro, who also won an Oscar for his performance, is lean and mean as boxing champion Jake La Motta. By the movie's end, he's nearly unrecognizable as the washed-up boxer.
Production on the film was done in two parts. Most of it, including all the boxing scenes, was shot first. Then production halted for several months while the king of method acting, De Niro, gained 60 pounds, reportedly by eating Italian food.
Director Martin Scorsese has said in interviews the scenes of fat Jake were shot quickly with few takes because of the physical strain on De Niro.
To portray a skinhead in the movie "American History X," Norton put on 35 pounds of muscle on his normally slender frame.
He has said he upped his protein intake to gain muscle. In an interview with Total Film, he said he spent about two and a half months getting into shape because he felt the character called for it.
"I think he's a character who's armored himself against the pain he's experienced in his life with his rage," he said. "I wanted him to have physically manifested that sort of armoring as well. When you talk to those kids, that's what those tattoos and muscles are all about. In a lot of ways, it's about creating a shield or a psychic empowerment device to deal with the feeling of being marginal and insecure."
The statuesque former model reportedly added 30 pounds to her 5-foot-9 frame by eating Krispy Kreme donuts and potato chips to play her Oscar-winning role as serial killer Aileen Wuornos in the film "Monster."
Even though the real Wuornos was much shorter than Theron, she had a toughness that Theron wanted to convey. With the additional pounds, makeup and prosthetic teeth, Theron was pleased with the result, she told The Advocate. "The first day we did all of it [hair and makeup] and I looked in the mirror, I was like [gasps], 'OK, I'm feeling this – this feels very authentic to me and very real and it's not a joke.' I was very happy."
When "Monster" wrapped, however, Theron had very little time to get back to her normal shape for her next film, which was starting production three days later. "I was training three, four hours a day, barely eating, trying to lose the weight – I only had three weeks till we were shooting – and I still had a lot of emotional baggage in me," she told The Advocate.
To play the emaciated insomniac Trevor Reznik in the dark suspense thriller "The Machinist," Bale lost over 60 pounds from his normally 6-foot, 185-pound frame.
"I had been to a nutritionist and when I had got down to what she had told me was a healthy weight, I just went: 'You know what? I can go more than this. I can keep going.' So I lost another 20 pounds below what she said I should stop at," Bale told the BBC.
No one was more surprised than his director, Brad Anderson, who expected Bale to lose only 10 or 15 pounds.
"But no, he's one of those actors, he needs to immerse himself in a role. He becomes the guy," Anderson told the Knight Ridder News Service.
Immediately after finishing "The Machinist," Bale went to the opposite extreme to play the lead role in "Batman Begins." In five months, he went from 121 pounds to 220 before slimming back down to 180 by the time shooting began.