Halliday (2008: 168):
Interpersonally, speaking is a two-way semiotic process, an ongoing exchange of meanings among different speakers; and analogously, spoken language is constantly shifting its deictic base, and alternating among the range of different speech functions. Writing is a one-way process (or was, until the coming of e-mail and then texting); and analogously, written language is largely invariant in mood and has a stable deictic base. While itself it is monologic, it can be turned into “virtual dialogue” by projection — which is another instance of semantic junction.