The most important is not what one would have to master. It is a simple principle but the most complicated concept created by cruel contemplation. What is it you mat ask? Permit me to refer you to  old fashion Black and White moves with no sound, The early movie makers had to convey a story without talking. Your minds would interpret the emotion of the Character. The eyes were the key. The actress communicated by stares, fluttering, glares, gazes, frozen or relaxed.

The walk, position and movement expressed fear, anger, amazement. A twisted open mouth represent a snarl. Arms  stretched palms facing the bad guy spoke, "Don't touch me." From these techniques have developed the concept of relating a person's mood. We call it, Show don't Tell."

Along with this we eliminate filter words such as -  felt, see, heard, looked, sound, smell, knew, thought, caused, decided, wondered, noticed, wished, realized and confusion. Variations encountered from tenses and genders broaden the range  of filter words. Avoiding filter words is called Deep POV which is part of SHOW. Can you completely eliminate filter words? NO! How ever if possible use them vert seldom.

The next set of words to eliminate are mood words. Each word may take a paragraph to SHOW. The emotion bless you with enhanced word counts and captivates your readers. It assists in what pro-writers call voice. Keep in mind, voice will suck you in without you noticing what is happening.Describe all the following in detail - excitement, frustration, anger, sadness, worry, fear. unease, nervousness, relief, surprise, happiness, jealously and embarrassment, (The word lists with a detailed explanation is found in S, A, Soule's, "The Writer's Guide to Character Development." All of her books on Deep POV are great."