A reaction shot is also generally bereft of dialogue, though this is not an absolute rule.
Its main purpose is to show an emotional response to the immediately preceding action or words of another character in the scene, or to an event in the immediately preceding scene which may or may not involve another actor (e.g., an explosion, monster, empty room, etc.)
A completely unemotional reaction may also be important if it provides information to the audience or is unexpected in the context of the scene.
In some cases, the deliberate avoidance of showing a reaction shot can be used by a filmmaker to dramatic effect; for instance, if the filmmaker wishes to conceal a character's motivations by hiding their reaction to a particular event.
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