The 35-year-old actress revealed as much in the May issue of American Way magazine, explaining she turned down the part of Vanessa because she didn't want play the girlfriend.
Vanessa instead went to Morena Baccarin, while Munn signed on to play Psylocke in X-Men: Apocalypse. Munn made sure Psylocke would do more fighting than talking before taking on the role, believing, "The fight scene could be her monologue."
"I thought Psylocke was always one of the most lethal characters and I said, 'Yes, as long as you're not using her to be the eye candy. She has really powerful abilities,'" the actress recalled. "And they said, 'Yes, that's an important part.'"
Psylocke, born Elizabeth "Betsy" Braddock, is a mutant with precognitive, telekinetic and telepathic powers, which often manifest as a "psychic knife." She later develops highly-level fighting skills.
"That was me from the beginning to end," Munn said of her stunts to Entertainment Tonight in February. "That was a really cool thing to feel like when I showed up on set I was like, 'Oh wait, I can do this! Throw me on a wire, give me a sword -- I'm good!'"
"Psylocke is such a powerful, lethal character," she explained. "It was important to me that I was able to do all of the stunts because the more believable it is, the better it is for the audience -- and the only way to make it believable is for me to actually do it."
X-Men: Apocalypse co-stars Jennifer Lawrence and Oscar Isaac and opens in theaters May 27. Munn is known for playing Sloan Sabbith on HBO series The Newsroom and is also slated for Office Christmas Party with Jennifer Aniston.