I actually have story to add to what Timon said about these fake characters interacting with us in our minds and at least partially becoming "real":
A few years ago, I was driving back home from a late evening class, and I was feeling very tired and all. Combined with the monotonous driving of being on a freeway that doesn't have any turns, my mind was left to wander.
Well it just happened that quite randomly, I got this very strong feeling that cub Simba was there sitting in the passenger seat of the car looking outside the window at the passing scenery. Even then I knew that couldn't be the case, but since the feeling was so strong, I still had to look to make sure that it was in fact not the case. (Although I would not have minded if he really was there I think)
I am still not sure if I should feel freaked out by that or not, but it wasn't anything serious and for a hallucination, if you can call it that, I think it was a rather pleasant one. The wonders of a bored and tired mind...