Halliday (2008: 164):
Interpersonally, probably the most significant variable in this context is that of personality, the different personæ being taken on by the writer and the speaker. Writing departs much further from the dialogic foundations of language. There have always been monologic forms of speech; but writing alters the balance, shifting from the intersubjective to the subjective, with the interesting consequence that the writer is, relatively, more first-person oriented than the speaker. If you are writing, your addressees are, typically, virtual; whereas if you are speaking your addressee is typically actual, and this imposes constraints on the meaning-making process: it has to be more a matter of negotiation. So while the writer’s personal intrusion into the discourse may be less apparent, because the mood tends to be constantly declarative, it is clearly present in the lexicalised systems of appraisal that we have become familiar with in recent systemic work.