The word. The sound of the word. The connotation and implication. The character. Amazing all the is contained in a word.
So, when writing dialogue you must consider her vocabulary and style. You must think about what he should reveal and has said before. Don’t be repetitive unless repetition brings a point home. Don’t, however, use different words that are forced. Make conversation sound natural even as it is cleverer than anything you would really say. Make the words brilliant but don’t draw undue attention. Use the dialogue to distinguish character–she would say this; he wouldn’t.
A good ear
and a unique voice.
Is that it?