You have to hand it to the stars of the Harry Potter franchise for making it to adulthood in one piece. Judging by the experiences of generations of too-much-too-soon ex-child stars, they should really all be drooling junkies, annoying party monsters or just flat-out dead by now.

But no. Having banked something like $10m each for surrendering a decade of their childhoods apiece, and having borne the weight of their generation’s pre-adolescent hopes and dreams, their post-Harry lives seem pretty normal and sane. Rupert Grint and Emma Watson appear well-adjusted and happy to take a lower profile.

Daniel Radcliffe is perhaps more ambitious about a post-Potter career. He telegraphed that early on by starring in a revival of Peter Shaffer’s play Equus – a demanding role with a lot of tabloid-friendly nudity – before he was even finished with Harry, and then doubled down with his work as young Allen Ginsberg in the proto-beatnik drama Kill Your Darlings.

All that Potter money has given Radcliffe the unique opportunity to bide his time, choose his parts, and get things right. In common with a lot of fresh-faced child stars waiting for the glow of infancy to burn off and the lines and wrinkles to build up – which can take for ever: see Leonardo DiCaprio, Kirsten Dunst et al – Radcliffe seems keen, understandably, to set fire to his cherubic younger self, establishing a firm demarcation line between all that and whatever comes next.

And what comes next is, this week, going undercover in a white-supremacist gang for Imperium. And, next week, playing a farting corpse.

Swiss Army Man is a vibrant desert-island allegory in which suicidal Paul Dano is saved when a dead Radcliffe washes up on his beach. I think we can safely assume that any movie in which Radcliffe plays a flatulent corpse whose erections function like a compass, and whose farts make him useful as a jet-ski and a weapon (like the knife, he’s multi-functional) is not aimed at JK Rowling’s global fanbase.

It’s certainly diverting. But it’s also an ambiguous, upbeat, finally rather moving story of self-delusion, madness and lost love. With Dano handling the wide-eyed, manic side of the story, Radcliffe makes a different kind of hay with his far from inert role. He deftly turns what could be a tedious acting exercise into a complex performance, full of life, despite being technically dead.

It feels like the final, decisive act of auto-infanticide for Radcliffe and Harry Potter: the Kid is dead; long live the King.

Swiss Army Man is in cinemas from Friday