Can you describe what insanity looks like to me?

Me and an actor friend were getting into a discussion about this. To the very detail, how would you stereotypically describe someone who is mentally imbalanced? We’ll say he’s sitting down.

Everything from a constriction of the pupils to the slightest movements of his feet. Expressions, twitches, tension, everything.

I know that usually you can’t tell a serial killer from a normal person, so just go with the stereotype in your head of a madman.