(3) where the "doer" IS relevant, for reasons of fact or to give the text the appropriate immediacy, the active voice works better.
I don't think it's quite that simple, because, the doer of the action can be specified in passive sentences, and there are active sentences where the doer of the action is not specified. For instance, these sentences are in the active voice, and the doer of the action is not known: The book fell off the table. The bread cuts easily. I'm afraid.
Mmm ... I'll agree that the doer might be highly relevant in a passive construction (it became clear that he was murdered by his wife), but I'll maintain that the active is usually better where we want to emphasise the doer (t became clear that his wife had murdered him / it became clear that it was his wife who had murdered him).
I'm not clear at all how the doer isn't stated in the other two examples:
In "I'm afraid", the person doing the being is "I" - right there at the start of the sentence.
In "the bread cuts easily", the bread itself (again mentioned at the beginning) is doing the action of the verb: in this case, the verb "cuts" does not take the meaning "wields a knife so as to make incision", it takes the meaning "is able to be cut" - and the bread is the one busy ising.
By "doer" I mean thematic agent - the participant that consciously causes the action to happen. That's a reasonable definition of "doer" I think, because otherwise we're just saying that the doer is the syntactic subject, which isn't helpful.
The bread cuts easily - who is cutting the bread? The book fell off the table - what caused the book to fall? I'm afraid - what's scaring me?
Note that passive versions of these sentences can have "the bread", "the book" and "I" as the syntactic subjects, but would you say that "the bread", "the book" and "I" were the doers?
The bread is cut. The book was pushed off the table. I am frightened by the monster.
> because otherwise we're just saying that the doer is the syntactic subject, which isn't helpful <
I'm not sure which way you're arguing there, goofy: if we're distinguishing between active and passive voice, then it is the syntactic subject, and only the syntactic subject, which determines it. If you're still arguing against my "where the "doer" IS relevant ... the active voice works better", then surely your examples support what I said: he pushed the bread off the table works better than the bread was pushed off the table by him.
(And I am frightened by is syntactically quite different from I'm afraid - the former has a hidden "doer" of the verb to frighten; the latter is subject-verb-adjective, and "I" am doing the being afraid.)
If the "doer" is the syntactic subject of the active sentence, then you're saying "where the syntactic subject of the active sentence is relevant, the active voice works better." But I don't think this is very helpful.
In my examples, the syntactic subject of the active sentences is the same as the syntactic subject of the passive sentences. This is because my active sentences use verbs that don't need the thematic agent as the subject. So according to your advice, the A sentences are better than the B sentences, even though the subject is identical in both.
1a The bread cuts easily. 1b The bread is being cut.
2a The book fell off the table. 2b The book was pushed off the table.
3a I was afraid. 3b I was frightened by a monster.
I could argue that the passive sentences are clearer because I can specify the thematic agents:
1c The bread is being cut by me. 2c The book was pushed off the table by him.
You might argue that these sentences are even better:
1d I cut the bread. 2d He pushed the book off the table.
But how are 1d and 2d different from 1a and 2a? They are both active. Your advice does not distinguish between them.
Wouldn't it be more helpful to say "where the thematic agent is relevant, specify the thematic agent?"