The film lives and dies on Chastain's shoulders, and she is extraordinary. As Ursula, she plays exhaustion without vanity - a woman whose love is real but rationed, whose patience has worn to a thread. As Other Mommy, she inverts everything: same face, same voice, but the warmth is wrong. Too constant. Too eager. Chastain calibrates the two performances so precisely that you catch yourself relaxing into the double before remembering what she is. It's the kind of work that could easily tip into camp, and she never lets it.
Clark is the revelation, though. Child performances in horror tend to split between catatonic and shrieking, but Clark builds Bela as a full person - watchful, lonely, hungry for attention in ways that make her vulnerability legible rather than merely convenient. The scenes between her and Chastain carry genuine emotional charge, which is precisely what makes the horror land.
Savage's craft has leveled up. Shot largely in tight interiors, the film uses negative space beautifully; doorways and hallways become threats without anything overtly supernatural happening in them. He's also smart about pacing. At 93 minutes, Other Mommy doesn't overstay - it tightens like a fist. The sound design deserves special mention: Other Mommy's presence is often announced by what's missing rather than what's added, a deadening of ambient noise that becomes Pavlovian by the midpoint.
The supporting cast rounds things out well. Duplass plays Russ with a quiet, deflecting guilt that makes him more frustrating than villainous - an accurate read on a certain kind of absent father. Dichen Lachman makes the most of a smaller role, Arian Moayed brings welcome texture as Uncle Frank, and Karen Allen's Grandma Ruth provides one of the film's few moments of genuine tenderness, which makes what follows hit harder.
Where the film stumbles is its climax. Without spoiling anything, Savage escalates into a more conventional confrontation that feels slightly at odds with the patient, psychological build he's constructed. There are one or two too many reveals stacked on top of each other, and the ambiguity that made the first two acts so effective gets partially resolved in ways that don't fully satisfy. It doesn't sink the film - the final image is genuinely chilling - but a more restrained landing might have elevated this from very good to exceptional.
