About half of the numbers take place in some sort of liminal musical reality whilst the others take place directly where the characters are in the moment.
spoiler
That's to show were Arthur begins dissociating and when the Joker instead of Arthur is fronting. Arthur and the Joker are headmates, they're distinct personalities sharing one body, and the difference between song and dance with a subdued instrumental score in the normal set vs full musical number with change of scenery has a purpose in narrating what is going on with the system that is constituted by these two headmates (and possibly more, although their existence is only hinted at). I have just seen the movie and haven't yet talked about it with a friend who's both a massive musical and film nerd and also plural, so i'm gonna withhold judgement on how well they handled this topic, but it's fairly obvious to anybody who's spent some time with people with plural experiences that Arthur and the Joker are supposed to be different ego states within a system. It's also expressly stated in the film even though it uses outdated language like "multiple personality" and Harvey Dent acts purposefully ignorant on what that means and how to recognize it, but it's very clearly shown as well.
I also think this shows how ingrained ableism is with the chuds who criticize the movie - you can show the Joker as a "mentally ill loser in a society that doesn't care" and they clap as long as they can hallucinate him as a fight club tough guy sigma, but when you go all in on what "mentally ill loser in a society that doesn't care" means, how forensic psychiatry and legal institutions treat a neurodivergent person, when you realistically show what is going on inside their head, when you detail the complex trauma in their past, when you compellingly show their weakness and violation and pain, these hogs cannot handle it.
spoiler
That's to show were Arthur begins dissociating and when the Joker instead of Arthur is fronting. Arthur and the Joker are headmates, they're distinct personalities sharing one body, and the difference between song and dance with a subdued instrumental score in the normal set vs full musical number with change of scenery has a purpose in narrating what is going on with the system that is constituted by these two headmates (and possibly more, although their existence is only hinted at). I have just seen the movie and haven't yet talked about it with a friend who's both a massive musical and film nerd and also plural, so i'm gonna withhold judgement on how well they handled this topic, but it's fairly obvious to anybody who's spent some time with people with plural experiences that Arthur and the Joker are supposed to be different ego states within a system. It's also expressly stated in the film even though it uses outdated language like "multiple personality" and Harvey Dent acts purposefully ignorant on what that means and how to recognize it, but it's very clearly shown as well.
I also think this shows how ingrained ableism is with the chuds who criticize the movie - you can show the Joker as a "mentally ill loser in a society that doesn't care" and they clap as long as they can hallucinate him as a fight club tough guy sigma, but when you go all in on what "mentally ill loser in a society that doesn't care" means, how forensic psychiatry and legal institutions treat a neurodivergent person, when you realistically show what is going on inside their head, when you detail the complex trauma in their past, when you compellingly show their weakness and violation and pain, these hogs cannot handle it.