When Noct is going through Zegnautus Keep and is travelling through the poison gas corridors, he starts hallucinating. Can be the other chocobros, can be Luna or Regis, or anyone really. Just really mess him up and make him question what's real or not.

Extra love if the hallucinations can't hurt him, but he panics so much he ends up hurting himself getting away from them (or whatever) and it blurs the line between illusion and reality even more.