In terms of tantalising Sundance prospects, the idea of watching Zac Efron play Ted Bundy has charmed its way to the top, morbid curiosity attaching itself to what screams of flashy stunt casting. The ex-Disney heartthrob graduated from the High School Musical franchise to a spotty adult career, showing comic skills in two sharp Neighbors films but struggling to make dross like We Are Your Friends, Baywatch or Dirty Grandpa feel remotely necessary.

His decision to play one of America’s most notorious and sadistic serial killers in Extremely Wicked, Shockingly Evil and Vile (named after a courtroom description of Bundy) is a carefully considered turning point, a gruesome gambit aimed at showing hidden depths underneath his Ken doll exterior and a textbook move for an actor whose work has been so closely associated with his appearance. But unlike other depth-plumbing heartthrobs like Jared Leto in Chapter 27 or Charlize Theron in Monster, Efron doesn’t require extreme weight gain or prosthetic teeth to transform, just an as-yet-unseen ability to lean into his dark side.

The film starts by showing us events from Bundy’s fiance Liz’s perspective as a single mother seduced by a charismatic stranger called Ted. The pair’s honeymoon phase is soon brought to an end when Ted is accused of aggravated assault and Liz is forced to decide between a growing stack of evidence and her lover’s dogged insistence that he’s being framed. The decision to keep the viewer similarly in the dark as to Bundy’s involvement in the crimes makes it a mostly bloodless affair and directed by the documentarian Joe Berlinger, it’s also a rather drab one too, betraying a devilish title hinting at wilder, unseen depths.

In fact, the most shocking thing about the film is Efron’s remarkably accomplished, fiercely committed performance. As Bundy, he ruthlessly weaponises the boyish charm that’s propelled much of his career, slyly convincing us of the spell he cast, not only on Liz but the many other women who were fighting his corner, sure of his innocence. It’s the career-changing moment he was clearly seeking and with an executive producer credit, one can understand his impassioned involvement, a juicy opportunity to break away from his pretty boy shackles and prove that he’s deserving of more dramatic work. But away from his standout turn, it’s a whole lot harder to figure out what those around him saw in the project.

Despite being centred on Bundy’s longtime partner, her on-screen incarnation is stuck in a rather repetitive and incredibly limiting cycle of worrying and drinking and Collins, channelling a mopey, mid-00s Jennifer Connelly, struggles to pull her out of the dark. She’s reduced to a stock character, which makes the choice to pull focus away from the gory details of Bundy’s crimes feel all the more bizarre. As good as Efron is, he too is saddled with playing a character short on detail. We know nothing of his childhood, his motivation or the minutiae of just how he got away with it all for so long. Berlinger, and screenwriter Michael Werwie, clearly want to put us in the place of Liz and the others he fooled by withholding even a glimpse of his violence until the last minute. But given that we know both his guilt and depravity before the film even starts, it’s a technique that doesn’t fully work. Instead, it results in confusing time leaps and a list of frustratingly unanswered questions.

Berlinger’s work as a documentary film-maker has brought him awards and acclaim for covering the crimes of everyone from Whitey Bulger to the West Memphis Three but his last narrative film was 2000’s disastrous sequel to The Blair Witch Project and it remains an ill-fitting format for him. As the festival kicked off, Berlinger also dropped a Bundy documentary on Netflix and at times, this feels like a lifeless reconstruction of that, only really coming to life when Efron is on the charm offensive. The facts of Bundy’s case remain fascinating and so they serve to drive interest regardless of presentation, but in order to separate this from previous attempts to bring his story to the screen, a surer, more stylish cinematic hand should have been pulling the strings.

It’s a star vehicle that starts and ends with its star, the film around him struggling to justify its existence. Efron is wicked, the film less so.

Extremely Wicked, Shockingly Evil and Vile is showing at the Sundance film festival