Papa Shango
Frontman of the WZ Band!!
Trust me, it can happen. When you play a character with zero empathy, no morals, not a single redeeming quality, it can make you sick. And I can see that happening with someone as dedicated as him. Apparently to keep himself in character, he started keeping a journal written as The Joker, writing down his thoughts while in character. I've never played a role that dark, but I have played some scummy characters, and trying to stay in character for extended periods of time is stressful, painstaking, and it does take a toll on you. It's one of the reasons I prefer comedy.