The actress dons a pixie-style hair cut for the role due to a key scene in the film.
"I wasn't expecting it to be such a big deal, because it was my idea," she said in an interview Thursday on Live! with Kelly. "It was something I've wanted to do for a long time, and I knew it was something the character did, so I just offered it to the director and he said 'Yes.'"
She said she was mentally prepared for the day of the cut, so she was surprised she reacted so strongly, People magazine reported.
"I've now done back flips out of windows, I've jumped off buildings," she said. "And cutting my hair reduced me to, like, mental patient-level crying. I was inconsolable."
A fellow actress did about half of the hair cut on film before Hathaway's stylist, Paul, had to stand in on film to do the rest while wearing the other actress' dress.
"It was all worth it because at the very end of the shot I turned around and I saw Paul, beautiful Paul, in the dress with his arms outstretched. It was totally worth it just for that," she said.
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