In particular, he has received plaudits for the way in which he captures the feeling of isolation experienced by those misunderstood by wider society.
But one man who is especially well-placed to appraise Phoenix's performance is Scott Lotan, who lives with a pathological laughing disorder much like Fleck's.
For Mr Lotan, who hails from Virginia Beach, USA, the pseudobulbar affect (or PBA) is a symptom of his multiple sclerosis which causes him to have laughing episodes that can last up to 10 minutes. As well as being exhausting and painful for the 47-year-old, the disorder often lands him in uncomfortable situations.
Speaking to LADbible, he said: "I have had issues with not being served at restaurants and been asked to leave because waitstaff were uncomfortable.
"Many times if I am out for a drink with friends, there is someone with low self-esteem that believes I am laughing at them and they will try and start a fight."
But Mr Lotan's PBA was never less welcome than in the aftermath of a tragic crash in 2003.
He said: "We were leaving my engagement party and were hit by a drunk driver. My fiancée died at the scene with my mother's death three days later.
"I remember being at the scene laughing and being questioned by police.
"At the wakes for both my mother and my fiancée I would have to separate myself from everyone as I would burst into laughter at times.
"I try to be fully aware of myself and I understand that it's beyond my control, but knowing that others think you are a freak and always explaining to people I am not this emotionally void psychopath can be difficult."
The recent release of Joker has shone a spotlight on the broadly misunderstood disorder, and while many viewers have reported feeling empathetic towards Joaquin Phoenix's Arthur Fleck, for Mr Lotan it was like looking in a mirror.
He said: "I think he did a great job of capturing the inability to stop laughing no matter what the circumstances are. I felt as if he experienced a deep sense of rejection in the bus scene, similar to how I felt during the days of my accident.
"It weighs heavy on the mind, people just look at you. You try and explain but they have preconceived notions that you are a drug addict or just a deranged lunatic. I think he captured the feeling of isolation and frustration with the lack of understanding from others.
"At times during the film I felt as though I was looking at a reflection of myself."
Mr Lotan added that Phoenix's portrayal of the disorder was accurate, right down to the uncomfortable choking during laughing episodes.
He said: "The choking comes from trying to catch my breath, desperately needing to breathe. Also, spit builds up in your mouth and as you pull in a breath the spit goes down the wrong pipe.
"My neck gets really sore and I am moving my head to try to relieve strain and sometimes that cuts off air."
Videos of Mr Lotan experiencing such episodes have previously gone viral, and while PBA has caused him a great deal of suffering, he is keen to emphasise that his life is not all doom and gloom.
He said: "There are a lot of comments of empathy [online], and although they are offering well wishes, they also seem to think I am living this horrible life filled with pain and suffering. Just like anyone else I have my ups and downs - sure, at times I laugh uncontrollably, but there could be much worse things to happen.
"For the most part it turns into a humorous thing when interacting with my kids."