The 63-year-old actor was asked by director Martin Scorsese to lose 20 pounds, reported Us magazine.
"Martin wanted us all to look a bit more gaunt, which (costars) Adam Driver and Andrew Garfield] did as well. I know Adam went to the extreme. He's an ex-Marine, and when he's given an order, he follows it through. He was like something out of Auschwitz," Neeson said.
Garfield also dropped 40 pounds and was engrossed in this transformation leaving a negative impact on his relationship with now ex-girlfriend Emma Stone.
The film is expected to release in 2016.