Even when skirting near autobiographical territory, there’s an important balancing act a writer must pull off between sharing their story with honesty and understanding what’s of interest outside of a very biased personal sphere. In Xavier Dolan’s wildly misfiring new film The Death and Life of John F. Donovan, his first English language feature, he revisits issues important to him and his work but with little care for what anyone else might find entertaining, engaging or even coherent to anyone but Xavier Dolan.

Before the film’s premiere at this year’s Toronto film festival, Dolan read out a letter he penned to Leonardo DiCaprio as an eight-year-old, anecdotal evidence of the inspiration for a plot which explores a similar, idealised relationship. As a 21-year-old, Rupert (Ben Schnetzer) is being interviewed by reluctant, snobbish journalist Audrey (Thandie Newton) about his new book, which recounts the letters he shared with actor John F Donovan (Kit Harington) as a child. We go back and forth to see Rupert as an 11-year old wannabe child actor (Jacob Tremblay) struggling to get used to life in England with a distracted mother (Natalie Portman) and John, struggling to hide his sexuality in an industry that isn’t ready for a gay star.

Stars were attached as early as 2014, soon after the release of Dolan’s most warmly received film to date, the Cannes-lauded melodrama Mommy. The cast also includes Kathy Bates, Michael Gambon and Susan Sarandon, with Jessica Chastain being left on the cutting room floor – a dazzling line-up of stars for any director, let alone one who has yet to turn 30. Dolan’s age has always been referenced in regards to his work and earlier on, this was understandable (after working as a child actor, he made his first film when he was 20) but it’s no longer enough to use his youth as a way of overlooking his repetitive excesses.

After his last film, the screechy, annoying It’s Only the End of the World, he’s descended even further into self-parody with Donovan, a film so befuddling to watch and understand that one can sense the tortured post-production process lingering over every scene. There are unavoidable sense-check issues with the set-up, which has an 11-year-old claim that he’s engaged in a secret letter-writing relationship with a Hollywood actor for five years. How a six-year-old is supposed to have engineered this is such a ridiculously improbable conceit, yet it’s one of many oversights in a rambling, often nonsensical script. The framing device is made almost immediately redundant when 21-year-old Ben admits that he didn’t really know what John was doing with a lot of his life, so any flashbacks are for events that he simply couldn’t have retold. It’s also clumsily used as a way of critiquing journalism, with Newton’s grouchy news hack being lectured by an actor about how important life outside of warzones is, especially when it involves listening to the naval-gazing pontificating of a cis white male actor.

Jacob Tremblay in The Death and Life of John F Donovan Photograph: 1996-98 AccuSoft Inc., All right/Toronto film festival

Dolan’s affinity for soap worked in his favour with the energetic earnestness of Mommy, but here it’s less well-orchestrated from the early grotesquery of Donovan’s family, including a maternal performance played to 11 by Susan Sarandon, to the cheaply wrought scenes of John and his secret lover. It’s so unclear what Dolan is aiming for here, a curious flatness pervading with a queerness strangely lacking and even his trademark pop aesthetic is barely seen, the rare sensory overload not landing as effectively as in his previous work. None of it rings remotely true and his insistence on playing out so many scenes at such a high level can make it an excruciating watch. It doesn’t help that Harington is such an empty presence. Admittedly his character is thinly etched but aside from aesthetics, he never convinces as an in-demand star, charisma failing to radiate. Newton has some tetchy fun, Bates has one solid monologue, Portman struggles with a character who can only really be defined as “mother” and Tremblay is stuck playing the same precocious kid role he’s becoming synonymous with.

It’s frustrating, given how Dolan does touch upon themes that could be interesting to explore, such as the difficulty of being gay in Hollywood and trying to figure out one’s own identity while living in the public eye, that there’s no depth to any of the film’s realisations – and so when he inevitably lurches toward a procession of sentiment, it feels false and unearned. We get a rainy reunion soundtracked as if it’s an insurance commercial, we get a bizarre cameo from Michael Gambon who may or may not be playing a ghost, and finally we’re lumped with a meaningless final scene accompanied by a song choice so head-smashingly obvious (an increasingly annoying trademark of his) that it starts to feel like Dolan is parodying a Dolan movie.

Near the end of the film, a character asks: “What should we know of an artist? What should he reveal to himself and why should it matter to us?” With The Death and Life of John F. Donovan, Dolan answers his own question in perhaps the way that he wasn’t intending to, none of this mattering to anyone but himself.