[...] Language serves what we may call an INTERPERSONAL function. This is quite different from expression of content. Here, the speaker is using language as the means of his own intrusion into the speech event: the expression of his comments, his attitudes and evaluations, and also of the relationship that he sets up between himself and the listener---in particular, the communication role that he adopts, of informing, questioning, greeting, persuading and the like. The interpersonal function thus subsumes both expressive and the conative, which are not in fact distinct in the linguistic system: to give one example, the meanings ʻI do not knowʼ (expressive) and ʻyou tell meʼ (conative) are combined in a single semantic feature, that of question, typically expressed in the grammar by an interrogative; the interrogative is both expressive and conative at the same time. The set of communication roles is unique among social relations in that it is brought into being and maintained solely through language. But the interpersonal element in language extends beyond what we might think of as its rhetorical functions. In the wider context, language is required to serve in the establishment and maintenance of all human relationships; it is the means whereby social groups are integrated and individual is identified and reinforced. It is I think significant for certain forms of literature that, since personality is dependent on interaction which is in turn mediated through language, the ʻinterpersonalʼ function in language is both interactional and personal: there is, in other words, a component in language which serves at one and the same time to express both the inner and the outer surfaces of the individual, as a single undifferentiated are of meaning potential that is personal in the broadest sense.
- Halliday (1973), a pag.106-107

[...] the macro-function that we shall refer to as the ʻinterpersonalʼ; it embodies all use of language to express social and personal relations, including all forms of the speaker’s intrusion into the speech situation and the speech act.
- Halliday (1973), a pag.41