The Thor star battled constant hunger pangs as he drastically slimmed down to portray a shipwrecked sailor for director Ron Howard's real-life maritime epic, which documents the sinking of a whaling ship in the Pacific Ocean in 1820 - the disaster which inspired Herman Melville's classic tale Moby-DICk.

"The physical transformation from the very beginning we all had somewhat of a goal we all wanted to get to, which was look as skinny as possible and beat to hell as we could possibly make ourselves look," Chris explains. "We started on one diet, normal 2,000 calories, and then each week we'd reduce that intake until the last couple of weeks where it was down to 500!"

The lack of nourishment caused Chris to experience a range of differing moods from one moment to the next, and his actress wife couldn't do anything about it.

"That's a pretty ugly sort of experience," he admits. "It led to some interesting mood swings and inconsistent emotion; many that my wife can vouch for, which were all negative!"

However, Chris insists he found some comfort as his co-stars, including Cillian Murphy, Ben Whishaw and Paul Anderson, were struggling through the same problems.

He says, "What was kind of great about it was we were all doing it together so it helped form this great bond and camaraderie."

Chris showed off the skinny physique he sported for the film in a post on Instagram.com on Sunday (22Nov15), which he captioned, "Just tried a new diet/training program called 'Lost At Sea'. Wouldn't recommend it.."

The actor confesses he "kind of loved" the ocean shoot for In The Heart Of The Sea, but it became particularly tough when they had to shoot in a freezing studio in London.

He reveals, "The hardest stuff was the tank in London because it was freezing on night shoots and it felt like a theme park from hell being shot with water cannons and flipped out of boats. Ron would be on the loudspeakers and we couldn't even hear! That was chaos. We thought the stuff on the ocean was gonna be trickier but the studio was more challenging with machines whereas on the ocean we just had to adapt to whatever the environment was doing."