Halliday & Matthiessen (1999: 524):
The most immediate way in which we act, grammatically, is through our choice of speech function. One kind of speech function is a command; this is obviously a way of getting someone to do something, but we tend to think of it as being in this respect untypical. However all speech functions are modes of action, whether command or offer, question or statement, or any of their innumerable combinations and subcategories. All dialogue is a process of exchanging meaning, in which the speaker is enacting, at any one time, a particular interpersonal relationship, including his own role and the role he is assigning to the listener (i.e. he is specifying a network of interpretations for his own and the others' behaviour).