Back in the day, I used to TA for undergrad English classes. I guest-taught a class or two every semester and ran tutorials, but far and away, the biggest part of my job was marking essays, and I marked hundreds of them over the years. My absolute biggest pet peeve and the thing I constantly lectured my students about? (No, it was not ending a sentence on a preposition, thank you very much.) Passive voice. I delivered many an impassioned classroom speech about the evils of passive voice, scrawled "PV" all over essays, and wrote a ton of comments on said essays explaining what passive voice is and why you shouldn't use it.
So what is passive voice? (Oh, I'm getting right back into TA-mode here!) Passive voice is a way of constructing a sentence whereby the object of the action become the subject of the sentence. In other words, the object of the action is the subject of the sentence rather than the person doing the action. For example:
Active voice: John scored the goal.
Passive voice: The goal was scored by John.
I hear you now: "ok, Sarah, it's a little awkward and clunky, I guess, but why is it the WORST?"
Here's why: It's not just that it's vague, but the reason behind using it is often because you're resisting naming who precisely is doing the action or you don't know the motivation behind the action. But if you don't know and you're the one writing it, how is anyone else supposed to know? "Mistakes were made." Ok. Who made them? How would it reflect on a character if you assigned him the blame for the mistakes?
As much as it pains me to admit it, passive voice doesn't always have to be the worst. For example, when obfuscation is necessary to the plot, passive voice can be a pretty great strategy. If you're writing a murder mystery, hiding who the killer is using passive ("The gun was fired from a close distance") can draw out the suspense. But in general, if you're writing fiction and lapsing into the passive voice, take a second to figure out why. Is it a conscious decision? Does it work for the scene that you're creating, or would putting it in active make it clearer and more direct?
Today is apparently Passive Voice Day (yes, it is kind of a thing), so celebrate for me by not using it! Please. PLEASE.