Unless you're writing specific types of stories, you generally won't use just one. "Show, don't tell" applies to some scenes and some words, but not all.
As with all writing advice, it's not one-size-fits-all, and "never" is a HUGE word in the writing craft.
TELLING: stating the "what" instead of the "how."
What is the character doing/thinking? What's happening around them? What are other characters thinking?
A great time to "tell" is when the detail's unimportant.
Ex: I found a blank piece of paper.
We don't need to know the sweeping expanses of whiteness across its thin, smooth texture.
You can also use telling to speed things up, move the story along, or summarize a period of time.
Ex: Over the next five years, I became a bumbling tangle of running late to everything and doing things just slightly wrong, not enough to warrant expulsion but enough to attract disapproving frowns.
SHOWING: portraying the "how," not just the "what."
This works best for depicting emotions and settings, but showing works well for stretching out a scene or building tension.
Showing a character's emotions rather than stating them puts the reader in the moment.
Telling: She was very worried.
Showing: Her head spun, and she staggered as her breaths grew short and ragged.
In most cases, showing and telling work together. One adds detail, and one keeps things moving.
Ex: Her head spun, and she staggered as her breaths grew short and ragged. Trembling, she curled up with her arms around her knees.
In short, showing vs. telling affects:
• tension buildup/release
You will hear from many well-meaning writers the phrase "Show, don't tell" or absolutes like "Never use 'said' or adverbs."
You own your writing. You're telling your story. What works for someone may not work for you, and vice versa.
To discern when to use show/tell, keep studying the craft of writing, which we're ALL apprentices in.
Be a storyteller. Increase tension where you'd want listeners to lean in, anticipating and on edge. Release and expand for that satisfying scene or short break.
(If you found this helpful, you might like my upcoming book I Can't Believe I Wrote That: A Self-Editing Guide, which you can add on Goodreads here: https://www.goodreads.com/book/show/43513578-i-can-t-believe-i-wrote-that).
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From an Editor: Common Writing MistakesNon-Fiction
As a content and copy editor, I see mistakes that many writers make. I've compiled these into some quick tips and examples for anyone who may find this helpful. (Adapted from a Twitter thread I wrote.)