Effective interaction among participants is crucial to the success of a presentation. The workspace of a presentation, which is the presentation slides, is a natural and effective medium of interactions among presentation participants. However, due to the insufficient support from existing collaborative presentation systems, workspace-mediated interaction is inconvenient in presentations, especially in collaborative presentations. In this paper, we explore typical workspace-mediated interaction forms and analyze their characteristics. Based on this analysis, we present a series of approaches to supporting workspace-mediated interaction in the context of the CoPowerPoint system.