Heath had a diary where he wrote notes and added images from Joker comics that he thought would help him build the persona of the role. He was also inspired by Alex from “A Clockwork Orange”, the character helped him form his own version of the joker.
In an interview Heath said “It’s a combination of reading all the comic books I could that were relevant to the script and then just closing my eyes and meditating on it.
I sat around in a hotel room in London for about a month, locked myself away, formed a little diary and experimented with voices - it was important to try to find a somewhat iconic voice and laugh.
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."