The complexity of spoken language is a grammatical complexity; spoken language constructs complex dependency structures (that is, elaborate edifices of parataxis and hypotaxis, out of both clauses and phrases), often accompanied by a relatively simple choice of words.
- Halliday (1985), a pag.330

[…] Spoken language responds continually to the small but subtle changes in its environment, both verbal and non-verbal, and in so doing exhibits a rich pattern of semantic, and hence also of grammatical, variation that does not get explored in writing. The context of spoken language is in constant state of flux, and the language has to be equally mobile and alert. This puts an intense semantic pressure not only on those systems that vary the form of the message, such as those of theme and information but also on highly-strung ones like tense and modality.
- Halliday (1985), a pag.XXIV

[...] much of what the written language achieves lexically is achieved by the spoken language through the grammar [...] The complexity of spoken language is more like that of a dance; it is not static and dense but mobile and intricate [...].
- Halliday (1985), a pag.XXIV