Halliday & Matthiessen (1999: 548):
The immediate appearance of order in written language — the fact that it is presented to us in neat blocks and rows upon a page (or the equivalent, in other forms of technology), whereas speech is notorious for its hesitations, false starts, backtracking, clearing of the throat and whatever — is simply a consequence of the fact that we do not display its history: we leave out the provisional attempts and early drafts, and "publish" only the finished product. When analogous measures are taken with spoken language there is no significant difference between the two: speech is just as orderly as writing (cf. Halliday, 1985/9).