Halliday & Matthiessen (1999: 525):
Choosing a particular speech function is, obviously, only one step in a dialogue; what the grammar creates, through the system of “mood”, is the potential for arguing, for an ongoing exchange of speech rôles among the interactants in a conversation. The mood system, together with other systems associated with it, constructs a great range of speech functional variation; and since in principle any ideational meaning can be mapped on to any interpersonal meaning, this makes it possible to construe any aspect of experience in dialogic form.