"Eating, eating, eating and eating," she told Access Hollywood of her new role. "As long as the costumes fit, I could eat as much as I wanted, which was great."
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The star said she gained some weight during the foodie film, though she didn't step on a scale to see exactly how much.
"It was a byproduct, yeah," she said. "There was some weight gain but I didn't weigh myself so I really don't know."
And Amy said she has a new appreciation for butter after seeing it in Julia's numerous recipes.
"I have completely reintroduced butter into my diet," she said. "I now cook my eggs with butter and it is just really, really good."
While her character, Julie, has some food phobias, Amy said she was much braver – until the lobster scene.
"I'm not afraid of the poultry, but I didn't really enjoy the lobster stuff," she said, laughing. "I went into it really fearlessly, like, 'Oh, I'm just gonna put a lobster in a pot' but I didn't go through the psychological journey of that for myself. I was supposed to do it and I couldn't."
Still, the lobsters went unharmed.
"They're real lobsters but there's no water," she said. "We switched lobsters out so no lobsters were getting horribly abused – for too long. [They] got better treatment than me!"
"Julie & Julia" is in theaters on August 7.