In their influential book "Public space" Carr et al. describe essential human needs that public spaces fulfill: (1) passive engagement with the environment, where we observe what others are doing; (2) active engagement through intellectual challenges posed by the space, or through engagement with the people in it; and (3) excitement of novel discoveries within the space. An often underused resource in public spaces - public displays - can be used to stimulate these needs. In this paper we argue for a new research direction that explores how public displays can stimulate such essential needs in public spaces. We describe and conceptualize related processes that occur around public displays, based on indepth observations of people interacting with a publicly fielded display application in a city center. Our conceptualization is meant to lay the foundations for designing engaging public display systems that stimulate PACD, and for supporting the analysis of existing deployments. Copyright © 2012 ACM.