I have yet to get cold called (two of my three doctrinal profs don't do cold calls), but I have to say that when my civ pro professor calls on others and they're not prepared, it is one of the most uncomfortable feelings there is. She doesn't yell, she just stands at the front of the room. Meanwhile, the silence in the class is absolutely deafening. Finally, after about a minute or two, she asks "Are you prepared today?" As if it wasn't obvious that the moron is not prepared. I think I'd much rather have a screamer.