Jellyfish, the people behind You Don't Know Jack (among other things), have published a nice, concise overview of principles for interactive conversation: The Jack Principles.

The Jack Principles are the result of 15 years of continuous research and development on Interactive Conversation at Jellyvision. A well-executed interactive conversation encourages you to suspend your disbelief and allow yourself to "feel" that a prerecorded host is talking, listening and intelligently responding to you. This illusion of awareness is the key to the success of Interactive Conversation.

The Jack Principles is the first set of comprehensive guidelines for designing, writing and performing for an interactive conversation.

[via the IxDA discussion list]