Halliday & Matthiessen (1999: 173):
Sometimes some entity can be construed in a figure either as a participant or as (an element in) a circumstance; in that case, construing it as participant means that it is being treated as more directly involved. Compare pairs of examples such as:
The pianist is more likely to escape unscathed as a circumstance than as a participant; likewise, the answer seems more impenetrable if guessed at than if guessed.