"And I was like, 'I'm not familiar with it. What does Wonder Woman really do?' And this person said, 'No, it's for Wonder Woman's mum,'" Theron said.
With Theron only 43 and Wonder Woman star Gal Gadot 34, the Tully star said the conversation is now a "defining" moment in her career.
"It was the defining moment when I crossed over," Theron said. "And I wasn't fully aware of it."
The role of Wonder Woman's mother Hippolyta in the 2017 movie eventually went to Connie Nielsen, who at 53 is a much more realistic fit.
In 2016 Theron toldVanity Fair Italy she doesn't like to talk about ageing in Hollywood as it reinforces the idea that "we are worth less when we age".
"How could I be afraid of something so natural?" Theron said. "Ageing is part of our life, we can't avoid that.
"And a part of me is really grateful for all the things that have happened since I started ageing; now I'm much wiser than 20 years ago. A wrinkle is nothing compared to that."
Theron's Wonder Woman revelation isn't the only surprising admission the actress has made while on the promotional trail for her new movie Long Shot.
Last month, the actress — who is considered one of the most beautiful women in Hollywood — revealed she was "shockingly" single.
"I've been single for 10 years, it's not a long shot," Theron told Entertainment Tonight. "Somebody just needs to grow a pair and step up … I'm shockingly available."
The star's Long Shot co-star Seth Rogen backed up her claims, adding: "She's out there!"
"I've made it very clear," Theron replied.