When confirmation landed last month that Warner Bros’ planned prequel to Willy Wonka and the Chocolate Factory was officially a go – with Paddington’s Paul King directing and Timothée Chalamet to star as the younger incarnation of Roald Dahl’s zany chocolatier – the news was as unsurprising as it was deflating. Origin stories are all the rage these days, and the idea of one for Wonka has been kicking around the industry for a few years now. Are people clamouring for it? Well, Hollywood franchises tend to run on an “if we build it, they will come” basis lately, so perhaps a wee Wonka adventure is just what the masses didn’t know they wanted.

Sure, Tim Burton and screenwriter John August attempted to forge a Wonka backstory – a whole lot of daddy issues, naturally – in 2005’s lavish but little-treasured Charlie and the Chocolate Factory, and nobody much cared for it. But that was then, back when Chalamet was still in short trousers: in Hollywood time, that IP has practically crystallised with age by now.

After all, this week marks a full 50 years since the first big-screen stab at Dahl’s 1964 bestseller was released, making Wonka an enduring figure of fascination for generations of children, while the ostensible hero of Dahl’s story – wholesome 11-year-old poppet Charlie Bucket – faded in his top-hatted shadow. Nobody’s ever expressed interest in Charlie-driven film spinoffs, while Dahl’s own sequel, 1972’s Charlie and the Great Glass Elevator, has remained unfilmed. It’s Wonka, the strangely sadistic man-child magician and devil-may-care capitalist, that people and producers can’t get enough of, and who can blame them? Wonka is weird and dry and inscrutable; Charlie is blandly selfless and inoffensive.

It was Mel Stuart’s 1971 film that stated this most bluntly, changing the title of Dahl’s beloved book to make Wonka the eponymous character. Half a century later, its legacy rests on Gene Wilder’s droll, eerily underplayed interpretation of Wonka, a sinister-sweet antihero who has haunted as many dreams as he has launched memes; much of the film around him is mechanical and twee by comparison, but you don’t remember those parts, so it hardly matters. Few films as spotty have attained cherished classic status on the strength of a single performance; on the flipside, it’s Johnny Depp’s off-puttingly fey, chilly spin on Wonka that has largely sunk the reputation of the 2005 remake by comparison, even though Burton’s film handily trumps Stuart’s for cinematic verve and vibrancy.

Dahl himself would be exasperated over the 1971 film’s endurance. Though he was nominally billed as its screenwriter, his original adaptation was scarcely detectable beneath all manner of uncredited rewrites, and he was vocal in his disdain for the result, Wilder and all. His list of grievances was long: Dahl had wanted the arch British peculiarity of Spike Milligan or Peter Sellers for Wonka, he was unhappy with the film’s foregrounding of Wonka over Charlie, he resented plot alterations and additions that muddied the cautionary neatness of his original tale, and he wasn’t a fan of Leslie Bricusse and Anthony Newley’s perky song score.

An author not caring for a creatively divergent adaptation of his book is hardly a stop-the-presses scandal, of course. But after watching Willy Wonka and the Chocolate Factory for the first time since my own childhood, where it was a VHS staple of 1980s schoolrooms and friends’ houses, I’m inclined to think he had a point. Stuart’s film is an odd, clunky beast, built from separate parts – a bit of Dahl’s updated Brothers Grimm misanthropy, a lot of cuddlier trends in 1970s family entertainment, and fading fumes from Hollywood’s blockbuster musical craze of the previous decade – that fit together as elegantly as Lego, Meccano and Play-Doh.

Stuart, a workmanlike film-maker hitherto best-known for documentaries and sitcom-like farces, directed it with a halting, gear-grinding rhythm and an erratic sense of pace: it’s a stately 45 minutes before Wonka even makes his first appearance, whereupon the film rushes through its fantastical factory set pieces with businesslike indifference.

The $3m budget was tight, and it shows, down to the all-too-detectable Munich location shooting. The much-vaunted factory’s production design is a feat of papier-mache resourcefulness rather than free-roaming whimsy, often looking altogether too industrial to capture the imagination: you never want to taste the screen the way Wonka invites his guests to lick the wallpaper. (Let alone the chocolate river: why is Augustus Gloop gulping down what looks like thin, mud-tinted dishwater?) As a fan of Dahl’s books, I was never quite taken with the film as a child, and now I see why: the book prompts mental images of near-impossible scale and extravagance, while in the film, you sense the corners and ceiling of every space.

Stuart’s film encapsulates what made Dahl’s children’s books, for all their zippy plottiness and vivid imagery, so hard to film. Almost any other creator’s attempt to wrestle with his singularly morbid, provocative storytelling sensibility – that gleefully cruel edge that so delighted unsentimental children even as it confounded many of their parents – plays as a compromise. (The Witches, Nicolas Roeg’s gleefully vicious, high-camp take on Dahl’s most terrifying book, came closest, only to bottle it with an altered, simplified happy ending that likewise enraged the author.)

And so the film honors Wonka’s antisocial, child-unfriendly coolness, to the point of leaving the fates of most of its young characters unresolved. (The book’s coda, detailing the kids’ survival, is excised from a rushed finale.) Yet it cushions him in starry-eyed romanticism, too, from the first bars of its signature song Candy Man, which posits Wonka as joy-spreading visionary who wants only to make the world taste good. Is he an idealist of pure imagination, or a canny commercial opportunist and coloniser of cultures? The film defused the NAACP-stoked controversy over the book’s racialised depiction of Wonka’s Oompa-Loompa workforce by making them orange-faced alien beings, but the sour taste remains.

Willy Wonka and the Chocolate Factory never decides who he is, and after half a century, the film – despite Wilder’s inspired, twinkle-toed turn – still doesn’t sit comfortably as a result. Will Hollywood’s next attempt to solve the Wonka riddle finally figure him out? Chances are Dahl will keep turning in his grave, though as he himself wrote: “You should never, never doubt something that no one is sure of.”