The Adelaide Festival of Ideas (AFOI), is a biennial festival of public debate on issues ranging from cosmology to art to public policy. It is one of the earliest of its kind, and the author has been involved on programming committees and as chair since its inception in 1999. This paper is an analysis of the sorts of public engagement the AFOI elicits from those who come to listen and engage. The sessions are largely free and well-attended, and the passive model of audience entertainment seems a very poor explanation of the civically active mode of engagement that actually occurs. This paper briefly maps the historical trajectory of the festival, and then uses Michael Warner's framework of publics and counterpublics to explain how the AFOI is both more and other than a simple addition to the entertainment industry. Understanding different qualities of engagement is a crucial issue for a broad appreciation of the public humanities.