Halliday & Matthiessen (2014: 225n):
The ‘Actor’ of a ‘material’ clause (Halliday, 1967/8) is distinct from the ‘Agent’ of an ‘effective’ clause … the two represent different generalisations about the experiential organisation of the clause. There is considerable variation in the use of the terms ‘agent’ and ‘actor’ in linguistics. For example, Dik (1978: 37) uses ‘agent’ (paired with ‘goal’) in a sense that is close to our ‘actor’, whereas Foley & van Valin (1984: 29ff) use ‘actor’ (paired with ‘undergoer’) in a sense that is closer to our ‘agent’.