Interaction is increasingly a public affair, taking place in our theatres, galleries, museums, exhibitions and on the city streets. This raises a new design challenge for HCI ? how should spectators experience a performer's interaction with a computer? We classify public interfaces (including examples from art, performance and exhibition design) according to the extent to which a performer's manipulations of an interface and their resulting effects are hidden, partially revealed, fully revealed or even amplified for spectators. Our taxonomy uncovers four broad design strategies: `secretive,' where manipulations and effects are largely hidden; `expressive,' where they tend to be revealed enabling the spectator to fully appreciate the performer's interaction; `magical,' where effects are revealed but the manipulations that caused them are hidden; and finally `suspenseful,' where manipulations are apparent but effects are only revealed as the spectator ...