Now here's a trick you can pull on an unsuspecting group of players. Have one of the players be secretly controlling one of their adversaries, unknown to the other players. The main difficulty is arranging it so that the GM can convincingly portray the antagonist as an NPC without arousing suspicion. Rather than go the detailed and clumsy route of passing notes for every single thing the antagonist says, it's better to have the GM and conspiring player meet for a bit to discuss general character motivations and directions, and then trust the GM to ad-lib within the parameters of that space.

This of course assumes the player you're conspiring with isn't a control freak.

Which might be okay, since usually in any game situation the biggest control freak will gravitate towards the job of being the GM anyway.

Transcript

Han: Wait... I've totally worked something out.
R2-D2: What?
Han: You might not believe this, but I think I know who this Darth Vader guy is!
Luke: The bad guy on the Peace Moon? Who?
Darth Vader: {Annie} That would be me.
GM: For reasons that will be apparent to some of us, who should stay silent at this point, Annie has been directing the actions of Darth Vader all along.
Luke: Wait, what?
GM: So now Annie can play Vader directly and stop passing me notes.
GM: And text messages.
Darth Vader: And emailed essays on character motivations.
GM: I wasn't going to mention those.
Luke: Let me get this straight. Annie, you're now playing Princess and her evil dad? How does that even work?
R2-D2: If I was controlling a bad guy you know what I'd do?
Obi-Wan: "If"?