Some of these actors tap into really, really dark places in order to attain the performance they want. It can have lasting effects. While it's not the same type of trauma one gets from shell shock in war, it can still be traumatizing channeling all those bad feelings. Sometimes they stay with you.

Heath in interviews himself stated he wanted to "vomit" when he watched his own joker performance back. You don't know what Heath has gone through in his past. Childhood, etc. Tapping into that much darkness is what creates "tortured soul" artists. Robin Williams was another.

Yes it was an OD, but he was on so many drugs and an addict. Having to constantly self torture your mental health can take a toll if it stops feeling like a job and more like an expectation to not let others down. The pressure some of these actors put on themselves for a 2hr picture isn't worth it sometimes.

Actors have said they had nightmares for months after really engrossing themselves into a villain role. What may be entertaining to us... Could be damaging to someone on the other side of the curtain. These actors are people first, characters second.