After the two plane crash survivors are rescued from the ocean, they are brought to a small concentration camp in the jungle. At one point, both men are brought out into the light and appear severely malnourished, with visible ribs and sunken abdomens. Were anorexic people cast as body "doubles"? Was this CGI (like Gollum)? Or was this accomplished another way?
According to this article,
In order to get the authentic look of POWs, Angelina put stars Jack O'Connell, Garrett Hedlund and Finn Wittrock on strict 700-calorie diets. While the dietary challenge presented obstacles, Angelina expressed her gratitude in a way that made it all worth it.
"The funny thing is we'd all be on set and everybody's kind of complaining and starving," Garrett said. "[We were] on these emaciated diets, and Angelina would show up on set and be like, 'You guys look great!'"
His prison-camp experiences alone, like those of Zamperini and other inmates, were harrowing, laden with starvation and abuse. Just to convey that, Magaro said the actors spent days covered in grime, including head-to-toe makeup resembling coal dust, and were thinned down considerably.
“There was a strict, strict diet that we were on, all the POWs as well as the raft guys. We had a nutritionist who kind of gave us a protocol and we followed it,” Magaro said of his three-month shoot in Australia in late 2013.
“It was a lot of cutting calories with a lot of cardio and exercise. I lost close to 35 pounds. I’m usually around 140 and I got down to 105. … It was difficult, but you have to remind yourself what story you’re trying to tell, and you want to do justice to the story. Everyone was very committed to it.”
So the actor did physically lose the weight with an extreme diet. There's no shortage of articles talking about how Angelina Jolie took on the same (or similar) diet out of sympathy for the actors and that's why she's so skinny lately.
There have been many movies where the actors have gone on crash/extreme diet to achieve the emaciated look. Like for instance Matthew McConaughey's Dallas Buyer's Club look.
Today, Christian Bale wouldn't need to drop a deadly 63 pounds for The Machinist. Digital retouching can take the physical element out of preparing for a role. For example, Jack O'Connell was saved from having to lose all the weight necessary to play a Nazi concentration camp prisoner in the Angelina Jolie-directed film Unbroken.