James McAvoy acts the hell out of 23 roles in Split, the story of Kevin, a psychiatric patient afflicted with dissociative identity disorder (DID). Actually, the actor introduces us to only a handful of these personalities. Too many "alters," as they're called, might spoil the brew cooked up by writer-director M. Night Shyamalan in one of his best psychological thrillers. In trying to repeat the success of his landmark 1999 scarefest The Sixth Sense, the director has backed himself into a lot of corners involving the mystical beings and surprise endings. Critical reaction has been cruel, sometimes justofied (Lady in the Water, The Last Airbender, After Earth) and sometimes unjustified (solid cases can be made for Signs, The Village and especially Unbreakable).
Split falls in the Shyamalan-plus column, mostly because McAvoy raises the bar on a banal girls-in-peril plot. After a high-school birthday party, popular Claire (Haley Lu Richardson) and Marcia (Jessica Sula), along with misfit Casey (Anya Taylor-Joy), are kidnapped by one of Kevin's alters and taken to a bunker-like cell. In the guise of "Dennis," head shaved and creepily confident, he tells the girls to strip to their undies in preparation for their sacrifice as "sacred food." They're all terrified. But Casey, with secrets of her own, has an agenda you won't see coming. Taylor-Joy makes good on the promise she showed in The Witch with a killer turn here. Her character can maneuver around Kevin when he shows up as Dennis or the chilling "Miss Patricia." But she is especially canny handling him as "Hedwig," a nine-year-old boy who maybe knows the secrets of getting the girls out of this hellhole.
This might have degenerated into a cheap gimmick if not for the way Shyamalan lets us inside the childhood trauma that pushed his tormentor into multiple personalities. It starts when Kevin, as fashion diva "Barry," swans into the office of Dr. Fletcher (Betty Buckley), clearly looking for help. (This veteran is such a glowing star of musical theater, from Cats to Grey Gardens, that you might forget what a superb dramatic actress she is.) This psychiatrist is meant to fill us in on Kevin’s condition – a sort of human exposition. Yet the stellar Buckley brings such warmth and conviction to the role that she allows us to see Kevin as a human being and not just a construct for tricked-up suspense.
And so we're off into a series of shocks and reverses that take a predictable but disappointing turn into the supernatural. Shyamalan can't stop himself. But through it all is McAvoy, playing these characters for real, with everything he's got, as if they meant something. Thanks to him, they do.