One cool thing you can do with NPCs in games is have them pretend to be someone else. If done correctly, the heroes should have no reason initially to doubt them, but as the adventure progresses they will come across strange oddities that don't make sense - until they figure out that the person is actually someone else.

For example, the old man living in the tower on the edge of the town says he's the local wizard. At first there's no real reason to doubt this, especially if the townsfolk back him up. But when things get tough and the heroes decide they could use an experienced wizard's help, the man turns out to be a fraud. (Idea totally not stolen from The Wizard of Oz, by the way - I just noticed the similarity after typing it up.)

The twist on this is if the NPC truly believes that they are someone else. This adds an element of convincing authenticity, because they can't be caught telling what they believe is an untruth, as such (so truth-detection magic or technology won't reveal the deception). They can still be found out, for example, if the "wizard" only ever does magic with the help of arcane devices, rather than spell-casting. And it can be interesting when confronted with the truth - the NPC can disbelieve it, or have some sort of mental breakdown as their delusion is shattered.

Now try this with the current ruler of a kingdom...

Transcript

Gonzo: Ha-ha!! You're too late! I've transmitted the first order to my agents. There's nothing you can do! Trust me!
R2-D2: That's the third time you've said that! You're not a clone of Darth Vader at all!
Gonzo: What? Of course I am!
R2-D2: No. It's all falling into place. You're a droid. Convinced you're a clone. Of Anakin. Under the control of Nute! You're just a puppet!
Gonzo: Droid? I am a clone of Anakin!
R2-D2: Who wasn't Vader! Vader was Padmé!
Gonzo: Noooo!!! That's impossible!
R2-D2: You're just a wannabe with a craving to emulate a villain with real spine - a myelo-yen! You're a Vader groupie!
Gonzo: Hey, I'm just trying to get in before it's cool.
C-3PO: A hipster Vader groupie.