Let’s put on a happy face, at least to start, for “Joker: Folie à Deux.”

If there’s one undeniably compelling thing about Todd Phillips’ 2019 original and his new follow-up, it’s that these movies are best when they dance. The first might have been a muddled attempt to retrofit a “Taxi Driver”-styled ’70s realism into a Joker origin story, but, man, when Joaquin Phoenix is on his toes, it’s hard to look away.

He’s nearly as captivating in “Folie à Deux,” a musical that closely follows the events of the first film as an imprisoned Arthur Fleck (Phoenix) goes on trial for the murders that occurred at the culmination of “Joker.”

“Folie à Deux” combines prison drama, courthouse thriller and musical, and yet turns out remarkably inert. If “Joker” — which some claimed sympathized the kind of lone gunmen that populate our real world — stirred debate, “Folie à Deux” is a self-conscious rejoinder to all that discussion, spending much of its time interrogating Arthur’s actions from the last movie.

That makes it a theoretically interesting film but a curiously dull one, particularly given that it stars two such incredibly watchable performers in Phoenix and Lady Gaga, who plays a fellow inmate, Lee Quinzel, infatuated with the Joker.

“You gotta joke for us today?” an Arkham State Hospital guard (Brendan Gleeson) asks as they pull Arthur from his cell. He is seemingly even thinner now. A wan look shows he’s jokeless, too, having clearly reverted back to the depression that Arthur earlier stewed in.

That interaction, and others that follow, carries on some of the themes of “Joker,” which imagined Arthur and the mania that springs from him as the warped product of a cruel urban world and failed social safety net. Arthur is now heading for either the death penalty or life in prison, it’s just a matter of whether his attorney (Catherine Keener) can convince a jury that he suffers from split personality syndrome.

Gotham City’s district attorney, Harvey Dent (Harry Lawtey), believes he should die for killing five, including the late-night talk-show host Murray Franklin live on air. Does Arthur deserve our sympathy? “Folie à Deux” is a little like the “Seinfeld” finale: a moral, courtroom rehashing.

The throngs outside the courthouse clamor not for Arthur but the Joker, whom they regard as an anarchist martyr. In many ways — including a mock Looney Tunes cartoon that opens the movie — “Folie à Deux” continues the first movie’s interest in considering, and satirizing, what it is we crave in entertainment. Do we want the “real” story of Arthur or the fantasy of the Joker?

It’s not surprising that the movie works best when Arthur and Lee lock into one other. This is Arthur’s first blush with the love he has lacked, but their connection may also have more to do with fantasy. Their time together is somewhat limited but, in Arthur’s imagination, their emotions soar in songs.

These musical interludes break free of an otherwise fairly bleak and belabored narrative, as a legal and penal system that doesn’t know how to handle Arthur’s pain — or that he’s a reflection of their failure — help twist him back into the Joker. Once the Joker does fully emerge, Phoenix’s Fleck is visibly aghast at what he has wrought.

All of this wrestling with “The Joker” makes “Folie à Deux” an impressively un-superhero-like movie and a deliberate denial of audience expectation. But it’s also spinning its wheels. It’s not surprising that “Folie à Deux” originated in concept as a stage show. It’s stuck in place, with only Phoenix’s dazzling contortions to marvel at.

MPA rating: R (for some strong violence, language throughout, some sexuality, and brief full nudity)

Running time: 1:19

How to watch: In theaters