Charlize Theron reveals the one snack she ate 'everywhere' while gaining weight for 'Tully'

Charlize Theron is a fiend for a certain processed snack.

The 42-year-old actress opens up on Tuesday's episode of The Ellen DeGeneres Show about her go-to junk food when quickly gaining 50 lbs. for her role in Tully, a movie about an overworked, overtired mother of three.

"I ate a lot of everything, but my drug of choice is potato chips," she confesss. "[I ate them] everywhere. I had a bag in the 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."

Theron admits she was a bit blindsided by the effects that excessive snacking would have on her mental state, adding that she felt "lethargic and tired all the time."

"The hardest thing for me was I wasn't prepared for how that amount of processed food will affect your mood, and I dealt with depression for the first time," she shares. "What they say about, you know, 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. That was a hard thing to break, because it's more mental than it is almost physical."

For her role as serial killer Aileen Wuornos in 2003's Monster, the star says she actually gained less weight -- and it was easier to lose.

"I was younger, I was 27, and I just didn't snack for a week and was back into shape," she recalls of the role that earned her a Best Actress Oscar win. "It was insane. That doesn't happen anymore."

As for losing the weight after shooting Tully, Theron recently told ET it "was hell."

"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 it was, like, prosthetics. And then I went on press junkets, and it was like nobody knew."

Theron did express to DeGeneres that she feels Tully -- in theaters May 4 -- is a pretty accurate portrayal of modern-day parenthood.

"The film deals with what parenthood looks like today. It's so insane to me that we live in a time where we share so much information. We share the medication that we take, whether it's through social media or whatever," she said. "But for some reason, when it comes to parenting, there's still so much stigma around it, and we just don't talk about a lot of stuff that parents go through. It's strange, like, I did this, yes. But mothers go through this every single day, and 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."

She added that being both an actress and a parent herself, she felt a responsibility to be as authentic as possible for the role.

"I feel like I had to do it for the film if I was going to play a mother who was having her third child," Theron added. "I felt a responsibility to that, and for myself, as an actor, I wanted to feel as much as I possibly could."

As for her own kids -- 2-year-old daughter August and 6-year-old son Jackson -- the movie star says "it's like war" at home.

"The last time I was here, the little one was a year-and-a-half, and the oldest was five. I didn't grow up with siblings so this is all new to me. Then, the older one would say, 'Let's do this,' and the little one would be like, 'OK!' It was a peaceful, beautiful, loving relationship that made me cry every single day," she tells DeGeneres. "Now I cry, but not because it's peaceful and loving anymore. The 1-year-old is now almost three, and she's like, 'No.' The older one is like, 'What?! What do you mean no?' It's like war in our house every single day. 'Don't touch me! Don't look at me!' 'I wanna be alone!' I'm like, 'Are they 13 already?' And one that says all of that with a lisp."

RELATED: Charlize Theron through the years

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Charlize Theron through the years
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Charlize Theron through the years
HOLLYWOOD, CA - OCTOBER 27: (L-R) TV personality/comedian Chelsea Handler (L) and actress Charlize Theron attend amfAR's Inspiration Gala at Milk Studios on October 27, 2016 in Hollywood, California. (Photo by Kevin Tachman/Getty Images for amfAR)
LOS ANGELES, CA - OCTOBER 27: Charlize Theron attends amfAR's Inspiration Gala Los Angeles at Milk Studios on October 27, 2016 in Los Angeles, California. (Photo by David Crotty/Patrick McMullan via Getty Images)
LOS ANGELES, CA - JULY 21: Actress Charlize Theron arrives at the premiere of DIRECTV's 'Dark Places' at Harmony Gold Theatre on July 21, 2015 in Los Angeles, California. (Photo by Axelle/Bauer-Griffin/FilmMagic)
LOS ANGELES, CA - NOVEMBER 18: Actress Charlize Theron attends the launch of UNAIDS New Fast Track Report ahead of World AIDS Day 2014 at UCLA on November 18, 2014 in Los Angeles, California. (Photo by David Buchan/Getty Images)
BEVERLY HILLS, CA- OCTOBER 17: Actress Charlize Theron arrives at the Wallis Annenberg Center For The Performing Arts Inaugural Gala at Wallis Annenberg Center for the Performing Arts on October 17, 2013 in Beverly Hills, California.(Photo by Jeffrey Mayer/WireImage)
Charlize Theron attends the Annual Charlize Theron African Outreach project at The Darby Restaurant on December 6, 2010 in New York City.
PROTECTIVE REPEAT: US actress Charlize Theron poses for a picture at the official grand opening party for the Atlantis Hotel on the Palm Jumeirah in Dubai on November 20, 2008. Dubai is threw a multi-million-dollar extravaganza today to launch a luxury hotel on an artificial palm-shaped island, despite the bite from the global financial crisis. More than 2,000 world celebrities are due to attend the event which is set to also include a firework display seven times larger than the Olympic display in Beijing, according to the event organisers. AFP PHOTO/KARIM SAHIB (Photo credit should read KARIM SAHIB/AFP/Getty Images)
NEW YORK - MARCH 11: Actress Charlize Theron attends the premiere of 'Sleepwalking' hosted by The Cinema Society and Dior Beauty at the Tribeca Grand Screening Room on March 11, 2006 in New York City. (Photo by Dimitrios Kambouris/WireImage)
HOLLYWOOD, United States: Best Actress Charlize Theron poses with her trophy at the 76th Academy Awards ceremony 29 February, 2004 at the Kodak Theater in Hollywood, CA. Theron won for 'Monster.' AFP PHOTO Jeff HAYNES (Photo credit should read Jeff HAYNES/AFP/Getty Images)
BEVERLY HILLS, CA - OCTOBER 16: Actress Charlize Theron attends the premiere of 'The Truth About Charlie' at the Samuel Goldwyn Theatre on October 16, 2002 in Beverly Hills, California. (Photo by Jon Kopaloff/Getty Images)
Actress Charlize Theron attends the 26th Annual People's Choice Awards on January 9, 2000 at Pasadena Civic Auditorium in Pasadena, California. (Photo by Ron Galella, Ltd./WireImage)
Actress Charlize Theron attends the 'Mighty Joe Young' Santa Monica Premiere on December 10, 1998 at Cineplex Odeon Broadway 4 in Santa Monica, California. (Photo by Ron Galella, Ltd./WireImage)
Actress Charlize Theron attends the '2 Days in the Valley' Westwood Premiere on September 20, 1996 at Mann Village Theatre in Westwood, California. (Photo by Ron Galella, Ltd./WireImage)
LOS ANGELES, CA - JULY 08: Actress Charlize Theron poses at the photo call For Focus Features' 'Kubo And The Two Strings' at Four Seasons Hotel Los Angeles at Beverly Hills on July 8, 2016 in Los Angeles, California. (Photo by Jon Kopaloff/FilmMagic)
LOS ANGELES, CA - JULY 08: Actress Charlize Theron attends a reunion for 'Two Days In The Valley' at NeueHouse Hollywood on July 8, 2016 in Los Angeles, California. (Photo by Alberto E. Rodriguez/Getty Images)
CANNES, FRANCE - MAY 20: Charlize Theron attends the screening of 'The Last Face' at the annual 69th Cannes Film Festival at Palais des Festivals on May 20, 2016 in Cannes, France. (Photo by Luca Teuchmann/WireImage)
CANNES, FRANCE - MAY 20: Charlize Theron attends during a press conference for 'The Last Face' during the 69th annual Cannes Film Festival, in Cannes, France, May 20, 2016. (Photo by Murat Unlu/Anadolu Agency/Getty Images)
CANNES, FRANCE - MAY 20: Charlize Theron attends the 'The Last Face' Photocall at the annual 69th Cannes Film Festival at Palais des Festivals on May 20, 2016 in Cannes, France. (Photo by Mike Marsland/Mike Marsland/WireImage)
CANNES, FRANCE - MAY 20: Adele Exarchopoulos and Charlize Theron attend the screening of 'The Last Face' at the annual 69th Cannes Film Festival at Palais des Festivals on May 20, 2016 in Cannes, France. (Photo by Luca Teuchmann/WireImage)
WESTWOOD, CALIFORNIA - APRIL 11: Charlize Theron arrives at the Premiere Of Universal Pictures' 'The Huntsman: Winter's War' on April 11, 2016 in Westwood, California. (Photo by Steve Granitz/WireImage)
WESTWOOD, CA - APRIL 11: Actress Charlize Theron arrives at the premiere of Universal Pictures' 'The Huntsman: Winter's War' on April 11, 2016 in Westwood, California. (Photo by Axelle/Bauer-Griffin/FilmMagic)
BURBANK, CALIFORNIA - APRIL 09: Charlize Theron arrives at the 2016 MTV Movie Awards at Warner Bros. Studios on April 9, 2016 in Burbank, California. (Photo by Steve Granitz/WireImage)
INGLEWOOD, CA - APRIL 07: Actress Charlize Theron walks the WE Carpet at WE Day California 2016 at The Forum on April 7, 2016 in Inglewood, California. (Photo by JB Lacroix/WireImage)
BEVERLY HILLS, CA - FEBRUARY 28: Actress Charlize Theron arrives at the 2016 Vanity Fair Oscar Party Hosted By Graydon Carter at Wallis Annenberg Center for the Performing Arts on February 28, 2016 in Beverly Hills, California. (Photo by Axelle/Bauer-Griffin/FilmMagic)
BEVERLY HILLS, CA - FEBRUARY 28: Actress Charlize Theron arrives at the 2016 Vanity Fair Oscar Party Hosted By Graydon Carter at Wallis Annenberg Center for the Performing Arts on February 28, 2016 in Beverly Hills, California. (Photo by John Shearer/Getty Images)
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