First things first: Split is not a film about mental illness, it’s a thriller.

Those hoping for Hollywood to finally do Dissociative Identity Disorder some semblance of justice, following decades of films featuring more than one personality per cast member, will be sorely disappointed. The linking of the mental condition with inherently widely disparate personalities and violent/sociopathic characters, in this case James McAvoy’s Kevin, if good fodder for a couple of hour’s entertainment like most films is anything but realistic.

Now on to the movie. Having achieved a return to form with last year’s marvellous back-to-basics shocker The Visit, Director M. Night Shyamalan’s much-anticipated Split, if thrilling, is, regrettably, not very scary. Similarly a bottle-shocker with characters you can count on your fingers, (not counting, of course, Kevin’s 23, and perhaps 24 distinct identities), Kevin’s personality ‘horde’ kidnaps three young women, holding them in anticipation of the arrival of something called ‘the Beast.’ Casey, the outsider amongst them (break-out talent Anya Taylor-Joy), attempts to communicate with any or all of Kevin’s personalities, while her fellow detainees try to break out.

With none among the three given too much opportunity for range beyond being perpetually frightened or facilitating the unfurling of Kevin’s many personas, the Morgan star nevertheless champions a deceptively complex character with which Shyamalan, in the film’s later stages, makes extremely strained attempts to draw parallels with Split’s central figure, without which the film would not be nearly as compelling.  Obsessed with pseudo-exploring the intricacies of Kevin’s condition rather than actually creating suspense or scares, of which there are few, with his psychiatrist Dr Fletcher (Betty Buckley) literally staging a Skype lecture to explain aspects of Kevin’s condition, Split falls into the trap perhaps inevitable in the case of Psycho of having to pause the action for lengths to explain just what the filmmakers are trying to achieve.

Today’s filmic audience however, unlike 50 years ago, are much more accustomed to psychological thrillers and speculative pop-psychology, and could easily have done without the lengthy exposition and therapy sessions; the backgrounder on Kevin and the less expected plot developments, more akin to science-fiction fare than a straight up thriller, being covered to great effect through a simple exchange between Kevin and his Doctor in the film’s penultimate stage. Similarly, Casey’s backstory, told sparingly through flashbacks and central to the film’s conclusion, could more than ably have been implied, if inexplicitly, through her reactions to Kevin and the others and the events of the film’s final sequence; Shyamalan’s chosen narrative device instead breaking up the tension and negating the promise of the kind of suspense-laden denouement evidenced in his earlier achievements.

Best when McAvoy is left to his devices or draws on his character’s comedic edge, his cycling through Kevin’s personalities over only a matter of minutes in one sequence offers a glimpse of what the film could have been should Split’s focus have been less on the implications of Kevin’s condition and more about the mesmerising interactions between the few cast members.

The surprise concluding scene, which will either produce a groan or a cheer depending on your view of Shyamalan’s body of work, and certainly the latter for this author, like Split’s trailer promises a great deal more than Shyamalan can necessarily, but will hopefully deliver in his, inevitably imminent, future projects.

Split is in cinemas now