HeadLock is a semi-automated system for head pose annotation that explores how human-computer interfaces can be combined with computer vision technologies to efficiently extract behavioral information from video recordings. For images with limited resolution, the orientation of a head is often the best approximation for gaze direction, a crucial component to analyzing the rich interactions and behaviors of humans. The goal of HeadLock is to reduce the cost of extracting head pose from video by several orders of magnitude by developing machine-perception technologies that can perform robust head pose estimation with minimal constraints on resolution and camera angle.