The actor, who stars as the title character superhero in the DC Comics project, was reading the script when he noticed the error in the famous Green Lantern Corps Oath.
He immediately stepped in to alert bosses to the mistake - because he knew fans of the original comic book would "murder" them if they got it wrong.
Reynolds tells Fandango.com, "I remember when I read... and there's been rewrites and redrafts and sometimes different dialogue is transcribed by people... and somebody wrote it down wrong. Nobody caught it and at the last minute I did.
"(I told them) 'You Cannot get the Oath wrong! We had a meeting about this... this is where (the fans) will murder you.' But it was amazing finally saying it for the first time."