Why has Joker: Folie à Deux divided critics?

Five years ago, the American writer-director Todd Phillips (until then best known for the Vegas-set comedy "The Hangover") won widespread acclaim for "Joker", a dark origin story about Arthur Fleck, the mentally ill, failed standup comic who commits unspeakable acts in Gotham City as his clown-faced criminal alter ego the Joker, said Peter Bradshaw in The Guardian.

In this strange "pastiche Scorsese", Fleck (Joaquin Phoenix) was reminiscent of Travis Bickle from "Taxi Driver" and Rupert Pupkin from "The King of Comedy", and got to kill Robert De Niro, who played both. I found the film laborious, but it made $1bn at the box office, and earned its star an Oscar. Now, the sequel is here, and though similarly tedious, it has been improved by the surprising addition of musical numbers.

"Eyebrows were raised" when it emerged that Joker was going to sing, said Robbie Collin in The Daily Telegraph. But actually, the American standards that punctuate the drama make a kind of sense: "each is a flight of fantasy that momentarily frees both character and film from their sombre shackles".

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The action is set in a home for the criminally insane, where Fleck is awaiting trial for murder, and in court; and the songs are performed either as solos or duets with fellow inmate Harley Quinn (Lady Gaga), who is infatuated with Fleck's alter ego. A quiet, "watchful" presence, Gaga is not given much to do apart from the musical numbers – but in them she is "creepily magnetic". As for Phoenix, he is "transfixingly unsettling", which adds to the tension.

The performances are good, said Christina Newland on the i news site. But in its attempts to explore issues such as mental health and abuse, the film is staggeringly inept. And Phoenix's "cartoon madman" act does start to pall.