Halliday & Matthiessen (1999: 191):
typifying rôles: active participant in figure of of doing [Actor…]
pronoun it/they; general noun creature; number category: count (singular/plural).
has potential for self-initiated action and movement [processes in which animal occurs as Actor are (unconscious but) voluntary; and may also be effective …]; also for perception [Senser in process of seeing and hearing].