To my memory, the Joker has always been a manic, sociopathic, mentally ill killer since his days in the comic strips, with a gun in one hand, and a knife in the other.
Watching Phoenix play him this way, in an alternate origin story, or character study is fantastic.
The throwback settings, the damp and dark streets, the crackling in your ear of gunfire, combined with real inner city struggles and realities, along with Joker's weeping, angstful laughs and cries of discontent and misery fill your senses.
I wont spoil the movie for you, there are plenty of other reviews for that, but I will say whether you're a comic book fan, a Joker fan, or a Phoenix fan, you wont be disappointed.
You will feel uneasy- as you should, you will not expect what occurs, and you will get much more than just a glimpse into how the circumstances, deceptions, and actions collide to create what ultimately becomes such a deep dark soul as the Joker.
Enjoy.