Remember who your targeted audience is. If you are writing a little kids book, make things brief, frank and simple.
But if you want a more 'mature' book, one that you would actually call, a 'book' or 'novel', then you need to act like you're writing for ages 12 and above.
No simple words. You can use them but try using other deeper ways to express them.
Example:
Zoey was sad because her hamster died.
and
Zoey's heart ached as she glanced at her now empty hamster cage.
Treat your audience as a smart audience. Don't be frank and say 'he died', instead write hints that have deep meaning, but are also obvious to potray the message you wish to show. Like 'he died' could become 'his heart stopped abruptly, causing him to go limp.'
And for more details, don't always use 'say', 'said' for every dialogue. Your readers are not dull, they want to know HOW the characters say things. Depressed? Filled with emotion? Joyous? Use these expressions to show how they speak.
"I got a new car!" He exclaimed.
compared to
"I got a new car!" He said.
It shows how happy he is, that he is raising his tone in excitement.
Said is flat, not tell revealing much about the thoughts and emotions.
Always leave hints, give them very obvious clues but never be too frank.
Treat your readers smartly and they will see you smartly as well.
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How to write with more detail
De TodoA book on good metaphors to use to give some details to stories.