Everyone's a character, so why isn't every sentence?
"If you do not see your main characters there in simple subjects, stated in a few short, concrete words, you have to look for them." Williams, page 43
Character and characters are very important in life. They're also pretty important to clearly identify in sentence structure. They aren't like deer or other game that many people enjoy hunting for. When the characters crucial to the purpose of the sentence are scattered randomly throughout a sentence, then the whole thing becomes opaque and unclear. People don't want to hunt for characters in sentences and don't enjoy those characters doing nothing active in a sentence.
Seeing as characters should most often be the central meaning of sentences, they need to fall in the subject so readers can clearly distinguish their critical importance. It's also not a bad idea to make that subject simple and clear. Don't hide it somewhere in the middle of a sentence to sound more academic. Most likely it will have the opposite affect and it's just a generally annoying thing to see.
When your characters are clearly distinguished, make them do something. Passive voice can become very redundant and monotonous. Too much of it indicates laziness or a general lack of effort from the writer. Can we 'is' or 'are' anything? NO! I can run, jump, play, read, write and any other litany of active verbs. Obviously writing differs significantly from the physicality of the active verbs actually doing something in real life, and there exist plenty of instances where linking and helping verbs are necessary. Avoiding their use altogether would actually damage your writing. But too many seem to have a sad reliance on them and that's terrible for your writing.
The characters are supposed to be reflective of reality. Their actions should be actions and as much as possible be written in an active voice. Make it simple and make it smart. Put your characters right out there and make them do something. You might find your writing will be doing something with that, too; improving.