if dealing with a psychotic character, then it will seem more fitting to mess with customers minds, rather than just run and jump. you can still use scaring tactics to make them scream, but i would go more with having the creepy factor play in.
walk in a slight limp, or drag your leg somewhat. pretend as if you've lost muscle control in parts of your body. sharp, fast twitches are very effective.
speaking wise, I would literally have conversations to yourself, in a whisper quiet style. shake and nod your head, as if agreeing/disagreeing with yourself. gurgling works alot too.
when talking to customers, begin with random questions, then tell yourself to be quiet and so on so forth (think of golem from LOTR, how he argued with himself). then you can use threatening lines, such as "noone can escape" or "welcome to my hell", or short lines.
giggling like a child creeps me out actually. perhaps trying that out? theres diffrent ways of going about being a psycho. it all must come from within your mind (no pun intended)