Why so serious? Joker: Folie a Deux trailer has Joaquin Phoenix and Gaga’s Harley Quinn doing musical numbers

Why so serious, as Heath Ledger famously uttered while playing the Joker in The Dark Knight, because the hotly anticipated sequel to Todd Phillips’ brooding 2019 film about the Clown Prince of Crime is, apparently, a fantasy romance musical.

That’s one takeaway after watching the first trailer for Joker: Folie a Deux, which Warner Bros sensationally dropped at CinemaCon in Las Vegas overnight.

Running at just under three minutes, the sneak peak gave fans a first proper look at Oscar winner Lady Gaga in the role of Harley Quinn - a very different portrayal of the character than Aussie superstar Margot Robbie’s - and provided an update on what happened to Joaquin Phoenix’s titular character after the events of the first movie.

That movie was the first film with an R-rating in the US to cross a billion dollars at the global box office, and it ended with Phoenix’s Arthur Fleck in Gotham’s Arkham Asylum.

But it wasn’t without controversy.

For a comic book movie, it tackled pretty serious themes of mental health, and some critics were upset at the sympathetic depiction of the misanthropic Joker character, expressing fears it could inspire real-life violence.

The controversy didn’t stop Phoenix winning an Academy Award for best actor for what was an incredible, transformative performance.

The actor, who most recently played Napoleon in the Ridley Scott historical epic of the same name, has transformed once more, with the trailer revealing Fleck is still in Arkham, only now he apparently has found a soulmate in Gaga’s Quinn.

Folie a Deux, in case you were wondering, is the medical term for a rare psychiatric syndrome, where symptoms of a delusional belief are passed from one individual to another.

From the trailer it’s reasonable to guess the title of the sequel describes a shared psychiatric delusion of Fleck and Quinn, but just how far that delusion stretches in the film is uncertain.

Set to the tune of What the World Needs Now, the trailer does feature fantasy musical sequences, although Phillips told CinemaCon earlier reports his film was a jukebox musical weren’t entirely accurate.

The trailer also appears to show Quinn as a patient in Arkham, which is a departure from the character’s traditional origin story as psychiatrist Dr. Harleen Quinzel.

So, how does Gaga’s Quinn co-exist in the same cinematic universe as Robbie’s? It doesn’t.

Joker: Folie a Deux is technically classified as a DC Elseworlds production, which means it exists outside of the new DC Universe, as imagined by new studio bosses James Gunn, director of the Guardians of the Galaxy franchise, and Peter Safran.

Matt Reeves’ The Batman reboot with Robert Pattinson and the coming HBO Penguin spin-off series, starring Colin Farrell, also fall under that banner.

Joker: Folie a Deux will hit Australian cinemas on October 3. Until then, try to put on a happy face.