Charlize Theron kept bags of crisps in her bathroom to help her pile on 50lbs pounds for 'Tully'.

The 42-year-old actress surrounded herself with packets of potato chips to make sure she put on weight to play mother-of-three Marlo in the forthcoming film, but eating large amounts of processed food left her feeling ''depressed'' and ''lethargic''.

She said: ''I ate a lot of everything, but my favourite thing, my drug of choice, is potato chips. I ate them everywhere.

''I had a bag in my car, a bag in my bathroom, a bag in the kitchen, a bag in the couch, a bag in my trailer - everywhere I went, there was a bag.

''I wasn't prepared for how that amount of processed food would affect your mood and I dealt with depression for the first time

''What they say about what you eat is kind of who you are is so true because I ate like a person who just didn't move and I felt like that and I was lethargic and tired all the time.''

Charlize felt a ''responsibility'' to put on pounds for the part, but admits it was tough to stop eating fatty foods once she'd started consuming copious amounts of calorific treats.

She said: ''That was a hard thing to break because it's more mental than physical. You can stop eating, but if you don't feel good about yourself then you don't feel like you can do it and it's hard.

''Mothers do this every single day. They get pregnant and they gain all that weight and it takes them a year and a half to lose it and if they don't they get judged.

''I feel like I had to do it for the film.

''If I was gonna go and play a mother who was having her third child, I felt a responsibility to do that and I feel like, as an actor, I wanted to feel as much as I possibly could.''

Charlize previously piled on 30lbs for her role as serial killer and former prostitute Aileen Wuornos in 2003 movie 'Monster', and believes she lost the weight quickly because she was much younger.

Speaking on 'The Ellen Degeneres Show', she added: ''I gained less but I think because I was like 27, I just didn't snack for a week and I was back into shape - it was insane.''