For Theron, gaining the weight was just as difficult as losing it.
"The first three weeks are always fun because you're just like a kid in a candy store. So it was fun to go and have breakfast at In-N-Out and have two milkshakes," she said. "And then after three weeks, it's not fun anymore. Like, all of a sudden you're just done eating that amount and then it becomes a job. I remember having to set my alarm in the middle of the night in order to just maintain [the weight]."
On Wednesday, she also appeared on The Ellen DeGeneres Show and talked about what exactly she ate to put on the pounds. "I ate a lot of everything, but my favorite thing—my drug of choice is potato chips," she told DeGeneres. "I had a bag in my car, a bag in the bathroom, a bag in the kitchen, a bag on the couch, a bag in my trailer. Everywhere I went there was just a bag."
This is not the first time Theron has had to intentionally fluctuate her weight for a role. She famously transformed to play serial killer Aileen Wuornos in Monster, which won her an Oscar. For Tully though, it took her longer to lose the weight after filming wrapped up.
"I was worried. I was like, this is taking a really long time," she said. "Because on Monster, I just didn't snack for five days and I was fine. You know your body at 27 is a little different than your body at 43, and my doctor made sure to make me very aware of that. Like, you are 42, calm down, you're not dying, all good."
She told DeGeneres that the mental aspect of the weight gain was just as difficult as the physical. "The hardest thing for me was I wasn’t prepared for how that amount of processed food will affect your mood. I dealt with depression for the first time. And so what they say about what you eat is kind of who you are is so true. I ate like a person who just didn’t move, and I felt like that," Theron added. "I was lethargic and tired all the time. That was a hard thing to break, because it’s more mental than it is almost physical. You can stop eating, but if you don’t feel good about yourself, then it’s hard."