WARNING photos of GBC ;p
It's the eyes - a particular dead stare seen in narcissists/psychopaths.
Worth knowing, for self-preservation reasons as much as anything else.
Hard to describe but to me there's a coldness and barely concealed rage there that makes them uncomfortable to look at.
Looking into those eyes, you'll probably find you get uncomfortable and look away after a few seconds, whereas with a normal person's eyes like Allison's you can look at just fine:
Even in this video (the infamous "little bit hurt") you can see that even though he's doing the fake sad act, the eyes keep the dead stare up
(cover the lower half of his face while watching and it's more apparent)
https://www.youtube.com/watch?v=c76CI6vtixI#t=29