Los Angeles -- James McAvoy must have jumped at the chance to star in Split.

His role, which involves playing a character with disassociate identity disorder (DID) and rapidly switching between different personalities, is surely a dream for any actor, and that's before you even come onto the fact it's a new M. Night Shyamalan movie he's starring in.

And McAvoy, at least, nails it. He breaks out his full actor's arsenal of ticks and affectations, playing some personalities with a twitchy humour and others with a blank creepiness that produces laughs and tension in equal measure. It works.

It's just a shame the film as a whole doesn't.

Splitrevolves around McAvoy's character Kevin kidnapping a trio of teenage girls and keeping them trapped in a gloomy underground room for ominous -- if initially unclear -- reasons.

The opening abduction sequence is Shyamalan at his creepy best. We watch as three girls -- Claire, Marcia and Casey -- leave the former's birthday party. Claire's dad is giving them a ride home. One of the girls, Casey -- played with an eerie blankness by Anya Taylor-Joy -- seems a bit odd, but we can't quite put our finger on why. The camera tracks Casey's haunted face as she climbs into the front seat of the car, while Claire's dad -- who lingers behind to shut the trunk -- is approached by an unseen stranger.

Everything about this scene works beautifully. The lingering shots, the unnerving atmosphere, the sudden explosion of violence -- all of these things blend together to create the kind of tension that made Shyamalan's earlier films so much fun to watch.

But sadly, it doesn't last. Although the film keeps up its suspense well into the third act, it's not long before the cracks start to show.

The film's female characters are a problem. They're well-acted, but they feel bland and underwritten. Casey's backstory unfolds in a series of grim flashbacks, but -- aside from offering an explanation as to why she's so withdrawn -- it doesn't really fit with the film's central plot.

Claire (Haley Lu Richardson) and Marcia (Jessica Sula), meanwhile, are entirely two-dimensional; they feel as though they've been written in purely to add some dialogue and a bit of suspense during their predictably unsuccessful escape attempts. Betty Buckley has a slightly meatier role as Kevin's therapist Dr. Fletcher, but again -- apart from a few entertainingly tense exchanges during their sessions together -- she doesn't have a great deal to do.

The other problem is Split's central concept. The idea that split personalities can develop their own unique physical characteristics that defy our knowledge of the human brain is an interesting one, but it really runs out of steam. The more we learn about Kevin's reasons for keeping the girls, and the more we find out about his ominous 24th personality, "The Beast," the less mysterious the film becomes. The tension leaks out.

And speaking of the film's final act, it's not giving too much away to say that there's an unexpected easter egg hiding towards the conclusion (this is an M. Night Shyamalan film, after all). As much as this semi-twist may appeal to many fans of Shyamalan's films, it also feels sort of cheap; it's like Shyamalan didn't quite know what to do with the ending, so he tried to crow-bar in something unexpected as a flimsy distraction.

Despite all of these issues, though, it's worth stressing that the film is still a lot of fun to watch. The tension might not be there for the full two-hour run-time, but it's there for most of it, and McAvoy really does shine; in fact the movie would almost be worth seeing for his energetic performance alone.

Shyamalan is ultimately a man of brilliant imagination and ideas, with a great talent for dialogue, atmosphere and suspense. We've seen what he can do when he's at his absolute best -- his name immediately conjures memories of Unbreakableand The Sixth Sense; it's a brand associated with some of the greatest supernatural thrillers of the last century.

But it's also a name that inspires a weight of expectations, and when the elements don't quite gel together -- as is the case with Split-- the result is all the more frustrating for it.