Halliday & Matthiessen (2014: 301):
Behavioural processes are almost always middle; the most typical pattern is a clause consisting of Behaver and process only, like Don’t breathe!, No one’s listening, He’s always grumbling. A common variant of these is that where the behaviour is dressed up as if it was a participant, like she sang a song, he gave a great yawn; this structure is typical in the everyday spoken language. The participant is analogous to the Scope of a ‘material’ clause (both being manifestations of the general function of Range); we shall call it Behaviour.