This paper presents an analysis of several recorded conversations and shows that dialogue utterances can be categorised into two main types: (a) those whose primary function is to impart novel information, or propositional content, and (b) those whose primary function is to relay discourse-related and interpersonal or affect-related information. Whereas the former have characteristics that are closer to read speech, the latter are more varying in their prosody and present a considerable challenge to current speech synthesis systems. The paper shows that these apparently simple utterances are both very frequent and very variable, and illustrates with examples why they present such a difficult challenge to current speech processing methods and synthesis techniques.