Originally Posted by OutRiddled
Plus, he also had to play Jack Napier as well and make his performance different to that when he 'becomes' Joker.
On this subject, look at the Joker before and after his transformation. It's kind of interesting.
Jack killed people.
Joker killed people as a means of artistic and psychological expression.
Jack was vain.
Joker killed vain people with their own beauty products.
Jack idly wondered about taking over from Grissom at some point.
Joker blew Grissom (and, later, all of his buddies) away.
Jack was scared of Batman.
Joker was captivated by Batman and nearly killed him.
Jack placed strong importance on loyalty.
Joker killed his #1 guy.
Jack was mildly amused by the TV news.
Joker shot the TV.
It seems that the Joker was basically who Jack always was deep down inside, but his plunge into the acid brought it to the fore. The Joker is Jack with all constraints lifted and with no pretense towards fitting in with the rest of society. Every idle thought or fantasy Jack may have had become top priorities for the Joker. There were no more restraints, the Joker simply did whatever he wanted.