Diction is the choice of words. Syntax is the arrangement of words. (Now I’m showing off but humor me. My MFA degree in fiction has put me in debt for the rest of my life so I ask for your understanding. Shall we continue?)
After getting words down on paper then begins the process of revision. Depending on what the story is about and how it’s to be told, go over this first draft (right after writing it or later) and take out words or add words that will get across to the reader not only what is happening in the story but also the feel of what is happening.
Arrange the words left on the page in the best order for clarity and maximum effect.
How important are diction and syntax? The words chosen and the order the words are in will alert the reader to how he or she should feel toward the characters in the story and how to feel about what is happening to the characters; and the reader will be alerted to how the writer feels toward the characters and the story, too. All this without any direct instructions from the writer.
Not having instructions from the writer is a good thing because the story should not be about the writer–unless you’re writing some kind of meta-fiction. The story should be about the characters. The writer should leave the characters alone. Let the characters “act out” the story without the writer sticking in an editorial nose and mucking everything up. Give the characters free will so that they act like real people even if the writer knows how it’s all going to turn out in the end.