Halliday & Matthiessen (1999: 523-4):
When we say that the grammar enacts interpersonal relationships, we mean relationships of all kinds from the transient exchange of speech roles in temporary transactional encounters (How are you? — Good, thanks; and you? — Coming along. Now what can I do for you?) to the enduring familial and other networks that constitute the structure of society. We tend to be less aware of this metafunction of language, at least in more learned contexts; partly because, as adults in a literate culture, we are conditioned to thinking of meaning purely in ideational terms (language as a means of "expressing thought"), and partly because it is less obvious that talking is a way of doing — of acting on others (and through them, on our shared environment) and in the process, constructing society. But the interpersonal and the ideational are the two facets of our everchanging social semiotic.