On the specific grammar of horror as opposed to thriller and why the distinction matters.

Horror and thriller are frequently conflated, especially in marketing materials. But they are formally and phenomenologically distinct experiences that produce different effects through different mechanisms.

A thriller produces anxiety: the tension of not knowing whether the protagonist will achieve their goal, escape the danger, or survive the situation. The threat is external and comprehensible. The pleasure is suspense and its release.

Horror produces dread: a sustained sense of wrongness that is not located in any specific threat and that does not resolve when the threat is removed. The threat in a horror film may be external and incomprehensible — a ghost, a possession, a force that violates natural law — or it may be internal, the discovery that the protagonist is themselves the source of the wrongness.

The best horror films use both mechanisms. Hereditary is a thriller in its first act and a horror film in its second, and the transition is the film's formal achievement. The moment when it stops being about what will happen and starts being about what is unavoidably true — that transition is when horror as a genre earns its distinctive power.