Generally, passive voice is weak writing, and something to avoid in good prose. However, this rule of thumb is not always true in fiction. Sometimes a character uses passive voice to denote a lower social position, or a villain might speak in such a manner to obfuscate their true motives.
The technical definition of passive voice means: "A verb is in the passive voice when the subject of the sentence is acted on by the verb."
This isn't very useful, is it? Technical definitions are seldom useful. Practical applications are best. So, here is an example:
Active: I wrote this blog.
Passive: The blog was written by me.
The passive sentence is wordy, weak and has a past form of the 'to be' verb in its structure. Active sentences are in the strong structure Noun+Verb+Subject. This structure is direct, short and powerful.
However, let's look at some characters in my novel Blood and Venom. In this scene, I have two maids gossiping about the Marquess of the estate. Ask yourself, would these characters use active or passive language in reference to such a powerful man in their lives? Obviously, the characters would speak in passive voice. Here's what MS-Word found wrong in the paragraph:
“Listen child…” The old maid dropped her voice to the quietest of whispers. “Not all are fired … some maids just … just … disappear. The pretty ones like you. Brown skin or not, I say, the lord of this estate is of the nature of Countess Bathory. It is his passion that makes the Wuthering Tree to mourn.”
“Hush,” Gracy sneered, “You prattle on about child’s fables. Do you think you can frighten me? I’m not moved by Ol’ Higue, duppies and such from my own land. I’ll not fear yours, neither.”
“I speak not of fables. I speak of the evil that births such fables to life. Was Vlad Tepes a fiction? Do not you have men’s whose hearts are bent on bloodshed in your island? Don’t you have pure evil there?”
Pretty Gracy stood thunderstruck for a moment. “Psswhaaww…” she finally hissed in disbelief.
“Aye,” the old woman raised a brow, “His eye is dark even to his own blood — that is, if she is his child at all. From the day she was brought here, he has never once been a father to her. I tell you plainly Miss Emily has run away to save her own life.”
MS-Word found two passive sentences in this dialogue, which gave me about a 10% passive voice for the prose. This was too much, according to a computer algorithm. Let's look at these two passive sentences and see if you agree with my inclusion of the passive structure:
In this example, the passive voice is definitely the best choice. The alternative does not make the sentence clearer.
Ol' Higue is the devil in Jamaica and duppies are ghosts.
MS-Word could not have correctly guessed that I am aiming for a Jamaican speech pattern.
Ultimately, I want to keep the reader focused on the Jamaican maid, so I put her identity first in the sentence structure.
The next suggestion my MS-Word is certainly a more powerful sentence in active voice. It wants me to change the passive voice:
"From the day she was brought here, he has never once been a father to her"
and make the sentence something like:
"He has never acted like a father to her."
Well, a maid would NOT be so direct and blunt in her words. A maid in Victorian England would carefully place the subject first, and hide the father in the sentence.
The father is a powerful Marquess, and a direct and immediate reference to him would be too harsh to the other characters.
So, you can see that passive writing is definitely a weaker form of prose. However, sometimes it is exactly the fact of that weakness that demands its use.