Charlize Theron gained a staggering 22 kilograms for a film role.

The star plays an overwhelmed mother named Marlo in her new film, Tully - a role which saw Theron eat around the clock in order to put weight on.

Speaking to Entertainment Tonight, Theron said: "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."

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She said she was eating In-N-Out burgers for breakfast and getting up to eat at 2am.

"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 2 in the morning and I'd have a cup of cold macaroni and cheese just next to me. 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."

She also said the weight gain was so shocking that her two kids, Jackson, 6, and August, 2, thought she was pregnant.

When filming was complete, Theron was then faced with the task of losing the weight again - a process which took a year and a half.

"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."