Abstract: Gestures’ expressivity, as perceived by humans, may be related to the amount of attention they attract. In this paper, we present two experiments that quantify behavior saliency by the rarity of selected motion and gestural features in a given context. The first one deals with the current quantity of motion of a person’s silhouette compared to a brief history of his quantity of motion values. The second one focuses on the motion speed of a person compared to the motion speed of other persons around him. Considering both features (speed and quantity of motion) and contexts (space and time), we compute a rarity index providing cues on the behavior novelty. This can be considered as a preliminary step to an expressive gesture analysis based on behavioral changes.