Whenever you see an exorcism scene, the demon is always angry. This is supposed to scare you, along with the wall-climbing, green vomiting and all of that. I don't know about you but this whole angry thing is losing it's appeal with me. After all, we expect the demon to be angry, right? And the whole scene is very predictable: priest comes in, demon is mad, priest splashes Holy water on the demon, demon goes away. Not scary.
Now, if as soon as the priest walks in, the demon gives him an uncaring look and continues whatever he's doing, you should be worried, because that's one psycho demon you have there. The kind of demon who after the priest commands him to depart, would look the priest in the eye and in his best DeNiro impression would say: You talkin' to me?
....now that's freaken scary.