Halliday & Matthiessen (2014: 147):
So, if we want to know why the speaker chooses this or that particular item as Subject of a proposition, there are two factors to be borne in mind. One is that, other things being equal, the same item will function both as Subject and as Theme. We saw in Chapter 3 that the unmarked Theme of a declarative clause is the Subject; so if the speaker wants to make the teapot his Theme, and to do so without the added implication of contrast that would be present if he made it a marked Theme (i.e. a Theme which is not also Subject, as in that teapot the duke gave to my aunt), he will choose an option with that teapot as Subject, namely that teapot was given by the duke to my aunt. Here there is an integrated choice of an item realising two functions simultaneously: Subject in the proposition, and Theme in the message.
At the same time, however, the selection of this item as Subject has a meaning in its own right: the speaker is assigning to the teapot not only the function of starting point of the message but also that of ‘resting point’ of the argument. And this is brought out if we dissociate one from the other, selecting different items as Subject and as Theme.