Transitivity isn’t a binary thing. You can scale it across usage. It’s scalar nature is readily apparent in much of lexical semantics. Consider the middle instances of φοβέω, for example.
- ἐφοβήθησαν δὲ ἀκούσαντες ὅτι Ῥωμαῖοί εἰσιν,
they became afraid when they heard they were Roman citizens (Acts 16:38). - ἐφοβήθησαν φόβον μέγαν·
They were greatly terrified (they feared [with a] great fear) (Luke 2:9) - ἐφοβήθησαν τοὺς ὄχλους
they feared the crowd (Mark 12:12)
- Intransitive change of state (grew afraid)
- → Semi-transitive cognate accusative experiencer-stimulus (were terrified)
- → Syntactic transitive experiencer-stimulus (feared X)
- → Semi-transitive cognate accusative experiencer-stimulus (were terrified)
Now from the perspective of the prototypical transitive clause (volitional animate participant acting on an inanimate non-volitional object), all three of these clauses are divergences. And the most transitive version of this verb necessarily involve the active voice:
- μηδὲν αὐτοὺς ταραχῶδες ἐφόβει
Nothing terrifying was causing them fear (i.e., “was frightening them” (Wisdom of Solomon 17:9)
Now transitivity is interesting in its own right, but there are additional implications here. When it comes to describing the semantics of the Greek middle voice, these kinds of patterns suggest that subject affectedness is not a sufficient means of explaining the Greek voice system. It might be a relevant condition for motivating middle morphology, but it is not the whole story. Transitivity plays a central role in all voice usage, not only in the contrast between active and middle (See Active+Reflexive vs. Middle Voice: What’s the Difference?), but also within the usage of the middle voice itself. Understanding that range is important for discerning the meaning of a given verb in the middle voice.
Rachel Aubrey’s paper at the Cambridge Greek Verb conference, published in the Greek Verb Revisited examines how metaphor and analogy effect language change in the middle voice with particular reference to -θη forms in the post-Classical era. And her larger thesis lays out an impressively comprehensive account of the Greek voice system.
Expensive Books that Deal with these Questions:
The Middle Voice by Suzanne Kemmer
Grammatical Voice (Cambridge Studies in Linguistics) by M. H. Klaiman
Prototypical Transitivity (Typological Studies in Language) by Åshild Næss
Rachel Aubrey’s thesis:
Hellenistic Greek middle voice: Semantic event structure and voice typology by Rachel Aubrey
Koine-Greek 