Halliday & Matthiessen (2014: 728-9):
In spoken language, the ideational content is loosely strung out, but in clausal patterns that can become highly intricate in movement: the complexity is dynamic – we might think of it in choreographic terms. In written language, the clausal patterns are typically rather simple; but the ideational content is densely packed in nominal constructions: here the complexity is more static – perhaps crystalline. These are, it should be made clear, general tendencies; not every particular instance will conform. But they do bring out the essential character of the relationship between the two. And it is the written kind of complexity that involves grammatical metaphor.