What he said in an interview. He also says it in that one for that magazine too.
"I sat around in a hotel room in London for about a month, locked myself away, formed a little diary and experimented with voices … I ended up landing more in the realm of a psychopath — someone with very little to no conscience towards his acts. He's just an absolute sociopath, a cold-blooded, mass-murdering clown, and Chris has given me free rein. Which is fun, because there are no real boundaries to what the Joker would say or do. Nothing intimidates him, and everything is a big joke."</p>