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."