The actress also spills some tea about her co-star Jason Momoa.
Heard and Fallon competed against each other in a "Spicy Bite Challenge," inspired by Heard's co-star Jason Momoa's children, who would spice up Heard's food while filming "Aquaman" to add some zing to the actors' strict superhero diet.
Fallon brought out two small plates that contained two tortilla chips -- one with a raw jalapeno pepper on top and the other with some hot sauce -- a thimble filled with liquefied ghost pepper, and a shot of milk.
The rules were bizarre, but simple. Fallon and Heard had to both eat the spicy bites while keeping their eyes locked on one another. The first person who looked away or took a sip of milk would lose.
Throughout the competition, Fallon and Heard tried to get each other to break, but we won't spoil the results for you.
During Fallon's interview with the actress, she offered more on-set secrets and explained why Momoa is "allergic to being ignored."
"He can't sit still," Heard said. "So no matter what, he will push, prod, nudge, shove you, anything to get your attention, especially if you enjoy a quieter activity, like reading and on set I like to read."
Heard revealed that to give himself "relief," Momoa would tear out the last pages of her books.
"Which is annoying," she said. "You get to like the last ten pages and they'd just be gone!"
"Aquaman" hits theaters Dec. 21.
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