Halliday & Matthiessen (1999: 172):
The different degrees of involvement are reflected in the way the figure, and its elements, are realised in the grammar of the clause. A participant is realised as a nominal group, and is typically placed next before or next after the verbal group realising the process. Circumstances typically occur further away from the process, and are of two distinct types. Type 1, simple circumstance, represents a quality; this type is realised as an adverbial group … . Type 2, macro circumstance, is realised as a prepositional phrase, which in turn consists of preposition + nominal group. The nominal group, as we have seen, construes an entity — something that could function directly as a participant Here however the entity is functioning only as a circumstantial element in the process: a location, or an instrument, or an accompanying entity, or so on (e.g. don't walk on the grass, I washed it with sugar soap, she came with her children); it enters into the clause by courtesy of the preposition, only indirectly so to speak.