The 42-year-old actress won an Oscar for her role and intense transformation in the 2003 drama Monster, and now Theron has completed another "very long" physical journey in her latest film, Tully.
ET's Kevin Frazer sat down with Theron at the movie's junket at the Four Seasons Hotel in Los Angeles on Tuesday, where they discussed how she gained weight for the role of Marlo, maintained it, and ultimately lost it after a year and a half.
"I gained close to 50 pounds for this film," Theron shared. "I just I wanted to feel what this woman felt, and I think that was a way for me to get closer to her and get into that mindset. You know, it was a huge surprise to me. I got hit in the face pretty hard with depression. Yeah, for the first time in my life I was eating so much processed foods and I drank way too much sugar. I was not that fun to be around on this film."
"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 explained. "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]."
"I would literally wake up at two in the morning and I'd have a cup of cold macaroni and cheese just next to me," she recalled. "I would wake up and I would just eat it...I would just, like, shove it in my throat. It's hard to maintain that weight."
As for how her two children -- 6-year-old son Jackson and 2-year-old daughter August -- handled her weight gain, Theron shared that they though she was pregnant.
"My little one was convinced that I had a baby in my belly," she laughed. "I had a very similar moment with my older one. Like in the film, where she saw my body one day and was like, 'Mom, what has happened to your body.' It was a little too close to home. I shared it with [director] Jas[on Reitman] and I was like, 'This is a little too real for me.' And [my kids] still refer to this movie as the movie where mommy had a big belly."
The actress ultimately lost the weight, but acknowledged that it "was hell."
"There's nothing fun about that. It took about a year and a half. It was a long journey, very long journey," she explained. "And it's hard because I had press junkets and movies around it and nobody knew that I had done it for this. And I think in the beginning everybody thought I was wearing, like, when the first photos came out, everyone [thought it] was, like, prosthetics. And then I went on press junkets and it was like nobody knew."
"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."
"The struggle is real for everybody," Theron stressed. "What's beautiful about this film, it's unifying. It makes you realize that nobody has it easier than you or they don't experience any of those things. It's messy."
She also explained that it's important for mothers to tell their husbands and co-parents how they're feeling.
"A lot of fathers have come up to me and said, 'I just didn't know. I didn't know,'" she shared. "I think that's on us. I think we should feel more comfortable to say, 'This is really hard and this is not what everybody told me it was going to look like. And why, when I say this, does it feel like it's handled with shame?' It shouldn't be. The stigma of what parenthood looks like; we should get rid of it."
This isn’t the first time Theron has opened up about struggling with motherhood. Watch below to hear what she's previously shared.