In Joker we can often see Arthur dancing (or doing some sort of approximation of a "clown dance"), but on a few occasions, this dance looks very forced and unnatural: his whole body is tense while his arms make some Tai-chi moves.
We can see it at least twice:
In the bathroom after killing the three guys in the subway and just before he enters the Murray show.
Is it possible that this is some sort of medical condition? OCD movement?
My interpretation of these dances is that he is expresses his emotions in the most direct and unfiltered way. These are the times where he is finally in touch with his inner self and the only way to express it is, well, with an expression-dance.
You can see the same thing with people under the influence of certain drugs or at least music. Some might even do it when they are very happy or sad. While the first dance is something like discovering his inner self for the first time and a gentle, yet affirmative slow-dance, the second dance is more of a excited and endorsing one that shows us a glimpse of the playful chaos he is bound to unleash.
