Words. Removing emotion words, that is.
She felt sad.
He was angry.
If you find you’ve written sentences like these in your manuscript, chances are good you have one of two problems:

• You’ve lapsed into telling the reader about the emotion instead of showing the reader what the character is feeling
• You don’t trust your ability to show your readers the emotion, and so you are ALSO telling your readers what’s going on, just to make sure they get it

Remove those emotion words, rewrite the section to SHOW the emotion, and you'll have a stronger story.

It’s easy to write, “He seemed agitated.” It’s much harder to think about and write something like:
His hands moved in a circuit from rubbing the back of his neck to pulling the collar of his shirt away from his throat to patting his right hip pocket. He flinched when the clerk asked for his order.
It’s more interesting for us readers to experience your story by reading these descriptions; we lose that experience when you tell us someone is bored.
We also get to know your character better when you show us how that character experiences boredom: one person may watch TV and eat a bag of chips; another may pace; and another may start picking at the loose threads on their sweater, slowly shredding it.
And, bonus!, these descriptions give you something more to work with later in your story. Perhaps that empty chip bag becomes a clue to whodunnit. Maybe that shredded sweater was a gift from an ex-boyfriend who comes back seeking a reconciliation.
When you come across emotion words in your manuscript, try deleting them. If the scene still shows the emotion, then you’re done. You didn’t need that crutch.
If you’ve lost something by deleting that emotion word, work on rewriting the scene a bit to show the emotion.
If you find you’re struggling with describing emotions, check out the Emotion Thesaurus. This book, written by writers for writers, lists 75-ish emotions and their definitions. The writers include both external (visible to an outsider) and internal (felt only by the person experiencing the emotion) signs that describe each emotion.

What do these sentences have in common?

​These sentences all use more words than are necessary to convey meaning. You can only nod your head,* and a nod only goes up and down. You can’t squint anything but your eyes, just as you can’t shrug anything but your shoulders. Ice is frozen; it’s always frozen. When ice isn’t frozen, it’s just … water. And when you kneel, you kneel down; you can’t kneel up!

​Adding those extra words creates a type of redundancy called pleonasm. As an editor, I also find those phrases pet peeves, and I kill them dead.

See what I did there? “Kill them dead” is another pleonasm. You can’t kill something halfway. (I did the same thing in the title of this blog post.)

Most pleonasms are stylistic errors, but you can use a pleonasm as a stylistic choice to add (poetic) emphasis:

​“I saw it with my own eyes.”
Whose eyes? My OWN eyes!

Some phrases, like “free gift” or “foreign imports” are used so often together that it can be hard to recognize that they contain redundancies. And some phrases, like "tuna fish" are idiomatic (in American English, at least), and so you can make a case that this instance is not a stylistic error. This list of 200 common redundancies in English can help you re-train your eye.

​When you spot pleonasms in your writing, it’s best to delete the redundancy, as many readers will consider it an error. Aside from that, deleting any redundancies just makes your writing tighter, which can help it become more powerful. And, if you’re focused on a tight word count, deleting redundancy is an easy way to start cutting.

​Alternatively, make very, very sure that you INTEND the redundancy because you want to emphasize the thing that is repeated.

* OK, so it’s not entirely true that you can only nod your head. James S. A. Corey, in his series The Expanse, created a hand- and body-gesture-based language for people who work in bulky space suits. You can’t see someone nodding in a space suit, so in his world, spacers “nod their fist.”