The easiest way to tell you about Joker: Folie a Deux is to tell you what it’s not.

It’s not a musical, despite heavy billing in that department, and a co-starring role by Lady “A Star Is Born” Gaga. Sure, there are songs, mostly from the middle third of the 20th century — Get Happy, World on a String, That’s Life, That’s Entertainment — but they tend to be warbled or sung-spoken, and to peter out just when they should be hitting their stride.