The actress lost a total of 25 pounds while making the film in order to authentically play Fantine, who sells her hair and teeth in order to survive.
Hathaway told Ew.com, "Before we started filming I lost 10 pounds and from the beginning of filming to when she (Fantine) is arrested, I had 14 days to lose 14 pounds, and I ended up losing 15. I lost 25 pounds total.
"I couldn't sleep. I was so starving, my body was keeping me awake at night, like it was telling me, 'Go look for food!' I was kind of in this otherworldly, slightly ecstatic manic state all the time.
"I had nothing else going on in my life, so there was no way to distract myself. At the end of a long day, I couldn't go home and comfort myself with a drink or with food. It was all Les Mis, all the time."
"Les Miserables" centers on the obsessive quest of Inspector Javert (Russell Crowe) as he spends years in an effort to capture escaped convict Jean Valjean. Hugh Jackman, Amanda Seyfried, and Sacha Baron Cohen also star.