Hasan (1985: 45):
If we define effectuality – or dynamism – as the quality of being able to affect the world around us, and of bringing change into the surrounding environment, the semantic value of the various –er roles must be seen as distinct. This distinction correlates with two factors:
(1) the nature of the Process configuration into which the –er role enters, i.e., what other transitivity functions there are within the same clause; and
(2) the nature of the carriers of roles, other than the –er role under focus ... a human carrier of –er role appears more dynamic than a non-human animate, and the latter appears more so than an object..
Halliday & Matthiessen (1999: 173):
The “degree of involvement” in the sense of how deeply some element is involved in actualising the process that is construed by the figure, can thus be represented as a cline: the difference appears not only between participants and circumstances as a whole, but also within each of these primary categories, so that there is a continuum from one to the other along this scale.