Halliday & Matthiessen (2014: 120):
But both are, of course, speaker-selected. It is the speaker who assigns both structures, mapping one on to the other to give a composite texture to the discourse and thereby relate it to its environment. At any point in the discourse process, there will have been built up a rich verbal and non-verbal environment for what is to follow; the speaker’s choices are made against the background of what has been said and what has happened before. The environment will often create local conditions which override the globally unmarked pattern of Theme within Given, New within Rheme.