It's the old adage that a hero is only as ever as good as their villains. After all, would Batman be as popular as he is without The Joker? John McClane without Hans Gruber? David Dunn without Mr. Glass?
Of course, creating a truly great villain is easier said than done. While having a visually interesting antagonist with a convincing motivation is part of the battle in creating a great foil for a hero, it's often the mystery of why they're doing what they're doing, and how they're keeping one step ahead of the protagonist, that makes them so fascinating.
Consequently, a lot of movie villains (even the ones mentioned previously), come to be defined by surprising plot twists and character reveals. Whether the writers decide to pull the rug out from under audiences and keep the real identity of an antagonist a secret, or throw in a plot point that completely reshapes their motivations and backstory, there are few things that can make a villain instantly memorable like a gut punch of a reveal that comes out of nowhere.