10 Subtle Details That Make Joaquin Phoenix's Joker Incredible
9. His Childish Tell
Eventually, we learn that Arthur's psychological trauma traces back to his childhood and abuse he suffered at the hands of his adoptive mother's boyfriends. He had his childhood robbed from him, in other words and then he became Penny's carer on top of that, feeding her even as he chose not to eat himself as they struggled financially. He's more like the parent than her.
The hint of his abuse actually comes a lot earlier than when he visits Arkham. The first time we see him watch Murray Franklin's show, he fantasises about being in the audience. But it's not just the "normal" Arthur who sits in the audience and ends up talking to Murray - it's Arthur as a child.
He talks about how it's his life-long dream to speak with him and to reinforce both that and the idea of Arthur never having developed fully beyond childishness, his demeanour is very much like that of a child. He softens and infantilises his voice, puts his hands behind his back nervously and swings his body slightly. In his dream world, he might look like an adult, but he's a child.