Look-at IK
An inverse-kinematics pass that turns the head, eyes or torso so a character follows a target with its gaze, layered on top of the playing animation.
An actor holds eye contact without thinking about it; a character whose head is locked in a recorded animation looks dead. Look-at IK fixes it after the fact: once the base animation has played, a chain of bones — often eyes, neck and chest — is rotated incrementally toward the target point, each taking a small share so the turn is spread naturally rather than wrenching the neck off its joint.
Each joint is constrained by an angular cone, so the character would rather lose sight of the target than spin its head 180 degrees. The technique runs as a layer after the ordinary animation, a cousin of foot IK but for the upper body, and is the cheapest trick for making NPCs seem aware of the player and of each other.