Joker: Folie à Deux is a terrible film, just not for the reason you'd expect.

Joker: Folie à Deux is what happens when a good idea meets flawed execution.

The film has only endured a short stint in cinemas so far, yet it's already been widely regarded as one of the worst releases of the year by critics and moviegoers alike, prompting people to ask how the hell it ever got made in the first place.

Yet if you look back at how its initial conception unfolded, all the signs pointed towards this project being a hit.

Joker: Folie à Deux is a sequel to Joker, the 2019 psychological thriller directed by Todd Phillips and loosely based on characters from DC Comics. The original movie starred Joaquin Phoenix as Arthur Fleck, a failed clown and aspiring stand-up comedian whose descent into mental illness led to a series of violent and murderous events.

Despite some debate and backlash surrounding the film, Joker went on to be the sixth-highest-grossing film of 2019 and the second-highest-grossing R-rated film of all time, as well as the first and only R-rated film to pass the billion-dollar mark (until Deadpool & Wolverine premiered this year).

The film won a slew of awards, including Best Actor and Best Original Score at the Oscars, and the creative team behind Joker were enticed to return for round two. Phoenix quickly signed on on to reprise the role that is seen as the greatest of his career, and soon, it was announced that the powerhouse that is Lady Gaga would be coming on board to put her own spin on the infamous character of Harley Quinn.

Joker: Folie à Deux takes place two years after the events of the first film. Arthur Fleck is in custody at Arkham State Hospital awaiting trial for his various crimes, and his lawyer is planning to argue that Arthur has dissociative identity disorder and that his "Joker" personality is responsible for the crimes, meaning he cannot be held responsible for his actions.

Watch the trailer for Joker: Folie à Deux. Article continues below.

During a music therapy session, Arthur meets fellow patient Harleen "Lee" Quinzel (Gaga), who is besotted with him. Throughout the trial, she drives him to embrace his Joker persona as his true self.

The strongest element of Joker: Folie à Deux is far and away the performances that Phoenix and Gaga deliver. Once again, Phoenix creates a layered, nuanced character of Arthur, making him chilling, sympathetic, sadistic, and pathetic in equal measure.

Meanwhile, the character of Lee is much more underwritten and underdeveloped, and faces the additional problem of being an already established and beloved character within the DC Extended Universe (most notably, she was played by Margot Robbie in the recent Suicide Squad and Birds of Prey films). Still, Gaga still manages to find a way to make the character her own, and her star power cuts through scenes where another actor playing Lee would have faded into the background.

And yet, despite the power of its leads, Joker: Folie à Deux is not a good movie.

Listen to Laura Brodnik's full review of Joker: Folie à Deux on The Spill.

The film's biggest downfall is that it attempts to be too many things, but slowly fails at all of them.

It flips between being a gritty psychological thriller, a thought-provoking courtroom drama, and a rollicking jukebox musical — but it never manages to stick the landing on any of these premises.

The biggest disappointment buried within Joker: Folie à Deux, though, is the musical element of the movie.

The idea to include musical moments in the film is actually an inspired choice, as it allows the audience to follow Arthur and Lee into their moments of delusion and fantasy — a world where they are wildly in love despite the fact that the prison is literally on fire around them.

And of course, it would also be a sin to cast Lady Gaga in your film and not utilise her voice. The scenes where Lee and Arthur sing to each other — imagining themselves in a world filled with elaborate stages and opulent costumes — offer an intriguing insight into their characters' minds, explaining their actions in a way the film's expositional scenes fail to do.

In a world where so many box office releases are falling into a pattern of sameness and safety, it was thrilling to think of the team behind this film taking such big creative swings and releasing a movie that would feel truly unique.

Yet they never quite commit to the musical element of the film, dotting just a handful of songs throughout the movie. The result is a film that never really pushes the boundaries in the way that we were promised, and it almost feels as if they shied away from the idea at the last minute. Instead, they opted for a more tried and true somber tone, rather than fully committing to the premise of a darkly unhinged musical.

Just before its release, there were whispers that Joker: Folie à Deux would be unwatchable because of its violent scenes, or that the musical element would drive people away.

Yet Joker: Folie à Deux's fatal flaw is not its violence, its premise or even its musical tendencies.

It's the fact that it's boring.

When it comes to movies, there are a multitude of sins that audiences will forgive if they're being entertained. So the fact that this Joker offering often feels like an endless slog to sit through is the real nail in its coffin.

While the story starts out strong with Arthur and Lee's first meeting, it quickly loses its momentum and builds to an unsatisfying conclusion.

Ultimately, it's an intriguing premise that, unfortunately, has been watered down into a story that feels like it didn't need to be told.

Joker: Folie à Deux is in cinemas now, it is rated MA15+

Laura Brodnik is Mamamia's Head of Entertainment and host of The Spill podcast. You can follow her on Instagram here for more entertainment news and recommendations.