Halliday & Matthiessen (1999: 190-1):
typifying rôles: active participant in figure of sensing [Senser…], of sayng [Sayer…] and of doing [Actor…]
pronoun he/she/they (also I/you); general noun person etc.; number category: count (singular/plural).
has potential for voluntary action [material: doing, including doing to another participant; verbal (semiotic): saying] and conscious processing of all kinds [mental: sensing, including feeling, thinking, intending as well as perceiving].