"It was very fun for me, because I was pregnant playing a drunk," she joked to ABC News. "I called [Nacho] and I said, 'I've got some great news for me, I'm pregnant.' He was ... so happy for me [but] after five minutes of just asking questions, he goes, 'Oh, wait a minute, can you do the movie?'"
Hathaway, 34, told the director she was game if he was too.
"He takes a split-second pause and goes, 'Your character's a drunk, you're bloated, it's perfect!" she said about his reaction.
As for how far along she was during filming, she said, "Not as early as you think. We got really creative by the end."
Minor spoilers ahead!
"It's not totally fun to exist in that type of mindset," he said, adding some scenes are "even still to this day [hard] to watch. But it all serves the story."
Hathaway said she was drawn to her character for several reasons.
"I really related to her and I thought a lot of other people would relate to her, this idea that you don't have to be perfect to be good and you don't even have to be good to be good," she said. "She just felt really real and really human to me, and I was excited to play somebody who had so many contradictions."
"Colossal" hits theaters this Friday.