Chris Hemsworth or Tom Hanks from "Cast Away"?

Answer: Hemsworth ... after he dropped major lbs for his starring role in Ron Howard's upcoming "In the Heart of the Sea."

Hemsworth plays a sailor stranded at sea in the Moby Dick-inspired flick, so he obviously got way into character. He posted the throwback pic from the set saying he wouldn't recommend the diet to anyone.

Hard to believe that's the same guy who plays Thor, but the good news is Chris is back to his standard beefy bod since filming.