Split depends on character work more than people sometimes give it credit for. The premise is high-concept, but the film only holds together if the people inside that premise feel tense, wounded, and specific.
Kevin Wendell Crumb
James McAvoy gives the film its engine. Kevin is not played as one simple villain. The movie keeps shifting between identities like Dennis, Patricia, Hedwig, and Barry, each with different rhythms, desires, and levels of threat. That constant instability keeps the captivity plot feeling volatile.
Casey Cooke
Casey is the emotional center of the movie. Anya Taylor-Joy plays her with caution, intelligence, and a deep reserve of pain that slowly becomes central to the story. Casey does not survive by being louder than anyone else. She survives by paying attention.
The other girls and Dr. Fletcher
Claire and Marcia help define the social world Casey comes from, while Dr. Karen Fletcher gives the film an important interpretive layer. Through her, the movie explores questions of identity, trauma, and belief about Kevin’s condition, even as it takes those questions in a heightened thriller direction.
The Beast hangs over all of this as a promise, a myth, and finally a physical threat. That slow build is a big part of the movie’s power.
The movie also gets extra mileage out of the fact that the supporting characters never feel interchangeable. Casey, Claire, Marcia, and Dr. Fletcher all respond to Kevin differently, which keeps the film from flattening into one long note of panic. Even when the script is operating in a heightened register, the human reactions stay varied enough to give the material shape.