A writer is said to be using passive voice when the subject of the sentence is who it happens to, rather than who dun it. Very occasionally this may be a good way to build up tension in the story, but as a general rule, it slows the story down and lifts the reader out of the action. Therefore it’s best to avoid using it.
The door was opened by the hero as he entered the room.
Seriously, that’s a pretty boring sentence. It’s passive.
The hero opened the door and entered the room.
Better still would be actions showing us how the hero entered the room. Most people open the door first. That's not exactly newsworthy. Did he kick it down? Slam it open? Peek around the corner first to see if the heroine was inside? Any of those would not only be active voice, but much more interesting to read.
Also they show us what happens instead of telling us, which is what the author should be aiming for.
The hero opened the door. He was angry.
Yes, so what.
The hero kicked the door open and raced into the room, fists clenched.
Ah, now we’re with him, wanting to read on.
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