Interactivity is a buzz word which is applied very broadly. It is helpful to try and look at different kinds of interactivity that you may wish to enage the user of your software with.
1. Forms of address:
2nd Person: The form of address used by television newscaster's when they directly address you the audience
3rd Person: You are on the outside, a "voyeur". watching a narrative unfold
*Computer Games often alternate forms of address between an active user mode to animated clips between levels.
2. Relationship type:
various group dynamics
3. Agents/impersonal interfaces:
An agent is an anthropormorphic software character who interacts with the user. Research into agents was developed at MIT Media Lab and Nicholas Negroponte's Being Digital discusses the concept in depth. The annoying little creature in Microsoft Word who offers to help you write letters is an agent.