The actor has received rave reviews for his portrayal of the first man to set foot on the moon, which also stars Claire Foy as Armstrong's wife Janet Shearon and is directed by Damien Chazelle.
"I actually didn't know if I was interested," Ryan admitted to the publication. "I certainly didn't feel like I wanted to do a biopic. But I think when I started diving into the materials, almost kind of by chance... the mission to the Moon, and the idea of what that mission meant and what it took and what it cost: that started really fascinating me."
The 37-year-old added that while he was aware of Armstrong's "contribution to history", there was plenty he didn't know about his life. And the filming experience, which included being strapped into a "multi-axis trainer" to simulate being in outer space, proved to Ryan that he wouldn't make a very good astronaut himself.
"It was...unsettling," he chuckled. "But, you know, kind of amazing. I realised I couldn't (actually cut it as an astronaut). Any fantasy I had about that was quickly shattered. It's a very specific person who wants to get into a machine that has never been flown, and fly it for the first time, and to actively push that machine to its breaking point, to see what its weaknesses are."