Having a character hiss dialogue when there s no s in it. A cold read by someone else will point out problems you might not see yourself. And if they don t automatically read it with the same inflections you intended, then you may need to re-examine how you wrote it. Use contractions In school, contractions are drummed out of us by well-meaning English teachers. But you re not writing academic papers.
Contractions are appropriate in both narrative and dialogue.
If you wish for a character to speak in a stilted way, then that might be a time to not use a contraction. You must, he insisted. It s a long story. First, I was headed to the mall when I spotted a dog running loose in the street. Not a collar but some electronic gizmo where a collar would be.