Talk about something (say, evil fairies) for long enough and the response turns from "Evil fairies, don't be ridiculous!" to "Evil fairies, I don't see them, where could they be hiding?"

Use this with a wide range of variants to ramp up paranoia. Have NPCs mention the werewolves that haunts the woods, or the gremlins that get into starship engines. A wide variety of people keep saying the same thing, despite no evidence ever turning up.

Keep it up until someone makes the critical turn of phrasing from "they don't exist" to "if they exist". Then soon after that something slightly odd happens. Not completely off the wall or impossible, but something that would be easier to explain if there were indeed active hostile forces at work.

Sit back and watch.

Transcript

Darth Vader: This is Lord Vader of the Empire. All Rebels lay down your arms.
Darth Vader: {over PA} Thank you, my daughter, for guiding me here. I shall rescue you soon.
Han: What?! I thought...
Leia: Ignore him! He's trying to confuse us and sow discord!
Han: But, but... I'm confused.
Chewbacca: Don't worry, Princess. We know you're not the traitor.
Darth Vader: {over PA} Some might consider you a traitor, but I know you are loyal to me.
Leia: That's a dirty lie!
C-3PO: Then who is the traitor?
Leia: There's no time! We need to get out of here!
{beat}
C-3PO: Artoo, I don't suppose there's space for me on the X-wing?