The 37-year-old actress discussed the movie on Wednesday's episode of The Ellen DeGeneres Show prior to attending the film's Los Angeles premiere.
"The film is about this pop singer who sort of is born out of a national tragedy where she, as a kid, writes a song in response to the tragedy and becomes this big star," Portman told host Ellen DeGeneres.
"Then she turns into kind of a monster and is a wild, wild woman," she added of her character, Celeste Montgomery.
Portman said she channeled her childhood icons, including Madonna and Tiffany, to portray the role.
"It was sort of like a kid dream of mine singing Madonna in front of the mirror with a hairbrush kind of growing up," the star said.
"When I was little, it was, like, Madonna. I had a Tiffany phase... showing my age. And then later on, 90s years was kind of more like Fiona Apple, Alanis Morissette," she added.
Portman wore a plunging blazer and matching pants to the Vox Lux premiere Wednesday evening. She was joined on the red carpet by Raffey Cassidy, who plays a young version of Celeste in the new movie.
FOLLOW REALITY TV WORLD ON THE ALL-NEW GOOGLE NEWS!
Reality TV World is now available on the all-new Google News app and website. Click here to visit our Google News page, and then click FOLLOW to add us as a news source!
Portman attended the premiere after apologizing to Jessica Simpson for her comments about an old magazine cover featuring the singer in a bikini. Simpson took offense after Portman told USA Today she was "confused" by the cover and Simpson's message about her virginity.
"@jessicasimpson Thank you for your words. I completely agree with you that a woman should be allowed to dress however she likes and behave however she likes and not be judged," Portman wrote on Instagram.
"I only meant to say I was confused - as a girl coming of age in the public eye around the same time - by the media's mixed messages about how girls and women were supposed to behave," she explained. "I didn't mean to shame you and I'm sorry for any hurt my words may have caused."
Copyright 2018 United Press International, Inc. (UPI). Any reproduction, republication, redistribution and/or modification of any UPI content is expressly prohibited without UPI's prior written consent.