Voices in your head is a good indicator of insanity. In games that have progressive insanity mechanics, such as Call of Cthulhu, you could try actually playing the voices in a character's head as a sort of NPC. For bonus points, do it in a way that only the insane player can hear. You could pass them notes, saying what the voices in their head are saying, or you can get creative with earphones.
Either way, some of the things the voices say will be mad ravings, but some of it could be coherent, and potentially offer insights that other characters don't get. The trick, of course, is to mix these up so that the player has no idea which is which...
Now, if you're a player and this is happening to your character.... what bits do you tell your companions?
Voice: Your grip on reality is as feeble as your excuses for your sins.
Palpatine: Leave me alone!
Voice: Alone? Alone is being abandoned and betrayed by everyone you ever trusted. Or loved.
Darth Vader: Everyone? No...
Voice: You are to blame! The Galaxy is to blame!
Voice: You will suffer! The Galaxy will suffer!
Voice: And then... you will be alone. With guilt your only company.
Luke: Whoever you are, you're clearly a force of pure evil. You should never have been brought into this world!
Darth Vader: How dare you!
Luke: Hey! I'm talkin' here! I'm talkin' here!