Unpredictability is your ally. If you know the only possible outcome is failure, that's no good. And if you know you're going to succeed, that's just too easy.
It follows quite simply: If you have no idea how your plans will work out, that's the best possible thing!
This lesson in roleplaying from the School of Jim.
Darth Vader: I see you've bought a new laser sword.
Luke: Actually, R2 made it.
R2-D2: It's artisanal.
Darth Vader: So you want peace and justice in the Galaxy?
Luke: That'd be a good start.
Darth Vader: You don't know what Palpatine is like. The demons of his past haunt him.
Luke: Well, show me. Take me to him.
Darth Vader: He is... unpredictable.
Luke: Then there's hope!
Darth Vader: Guards! Take him to my ship.
Clone Trooper 1: Uh, okay, Sir.
Clone Trooper 2: Pssst. "Ma'am."
Clone Trooper 1: Sorry. This way, ma'am.