I posted somthing like this a while ago,
The joker has a condition known as "Labile effect" look it up, and a "Chealsey Grin" or "Glasgow Smile"
wich is when people would cut they're victums faces unto a smile, then kick them in the groin or stomach so they're scream would rip into an even more painful, deadly cut.
Plus there is a drug known as "street valum" but it is really Haldol, which can make your face frozen into a smile.
video of the effects of haldol-