For me, it's knowing the different levels to emotions and as an author, knowing the right "degree" to the emotion. *This is important since like people, your characters won't all show emotions the same way.
For example Character A could be jealous that Character B is seemingly flirting with C. The obvious way to show this emotion from A because angry and "green with envy". But if we understand A, they could also show this emotion has sad/withdrawn, etc.