Hathaway would eat only two thin slices of dried oatmeal a day during the shoot and ended up losing 11kgs, reported Daily Express.
"I had to be obsessive about it - the idea was to look near death. Looking back on the whole experience - and I don't judge it in any way - it was definitely a little nuts.
It was definitely a break with reality, but I think that's who Fantine is anyway," she said.
When filming finished, Hathaway said she was in "a state of deprivation - physical and emotional."
"When I got home, I couldn't react to the chaos of the world without being overwhelmed. It took me weeks till I felt like myself again," she added.
The film has been directed by Tom Hooper of 'King's Speech' and stars Hathaway, Hugh Jackman, Russell Crowe, Amanda Seyfried, Helena Bonham Carter, Sacha Baron Cohen and Eddie Redmayne.