Halliday & Matthiessen (2014: 30):
At the same time, whenever we use language there is always something else going on. While construing, language is always also enacting: enacting our personal and social relationships with the other people around us. The clause of the grammar is not only a figure, representing some process — some doing or happening, saying or sensing, being or having — together with its various participants and circumstances; it is also a proposition, or a proposal, whereby we inform or question, give an order or make an offer, and express our appraisal of and attitude towards whoever we are addressing and what we are talking about. This kind of meaning is more active: if the ideational function of the grammar is ‘language as reflection’, this is ‘language as action’. We call it the interpersonal metafunction, to suggest that it is both interactive and personal.