A character's thoughts are possibly their most private and personal thing, so invading them represents a severe violation of character integrity. This can represent things such as horror and encroaching madness, or the invasion of mind controlling aliens, or magical spells that change your behaviour.

Next time a PC gets a simple Charm Person spell cast on them, try representing it by informing the player that their thoughts are being drowned out and overridden from outside. Bonus points if you physically get up and whisper horrible sibilant things in their ear.

Transcript

GM: While this is happening, Luke, your thoughts become consumed by phrases you've never heard, thoughts you've never had, rushing in a kaleidoscope of fractured life force.
Luke: Whaa?!
GM: One familiar voice pierces the cacophony.
Yoda's voice: Afraid, do not be!
GM: The sensation diminishes and you find yourself in control of your body again.
Yoda's voice: Driven Anakin out, we have.
Chewbacca: Huh. You've been midi-chlorinated.