Show don't tell is one of the basic rules of writing. It means that you should never tell a reader what your character is feeling. Instead, show your reader how the character is feeling by describing his actions. You shouldn't write "Jack is angry" but you could write that Jack "clenched his fists" or "gritted his teeth" and your reader would understand these visual clues as meaning that Jack is angry. Think about how an angry or sad person acts. You wouldn't write "Sally is sad" and you shouldn't write "Sally is crying" either, but you could write "Sally's eyes were puffy and bloodshot. Her shirt sleeves were covered in wet mascara streaks."
Reveal how your character feels about something with dialogue. If you want your character to be emotional, have another character say something provoking to them. For example, if your character is afraid of commitment, have her boyfriend bring up commitment. If you use realistic dialogue your character will avoid the conversation, get cornered into it and then become increasingly annoyed until she says something hurtful like, "You just want to get married because you're insecure" or, "You want to tie me down because you know I'm too good for you."
You'll never actually say that she's afraid of commitment, but the reader understands that she is because of the way she reacted to the conversation. Do some research into human psychology and you'll discover how coping mechanisms and avoidance tactics factor into human behavior.
If you're using first person narration, the reader will be inside your character's mind. In this case, think about what goes through your own mind when you're emotional. If you're angry you don't think "I'm angry" but you probably blame other people, use curse words, become irrational and maybe even want to physically attack someone. Use this in your character's stream of consciousness.
If you use third person narration, you can describe a character's looks from the outside and how other characters react to them. For example, if a character is angry, another character might react to them by cowering, running away, or yelling back. If you're using third person omniscient narration, your reader will be able to read the minds of other characters. Use omniscient narration to your advantage by giving your reader a biased opinion of a protagonist's emotional state as interpreted through the eyes of another character.
Assuming that your audience is smart means that you assume they will be able to read between the lines. They will pick up on small clues and piece things together for themselves. Feeding them too much information will ruin suspense and create boredom. Writers overfeed their audience by overusing adjectives and adverbs. Adjectives are unnecessary if you choose a strong enough noun and same goes with adverbs. You don't need to say that Jack "slammed the door angrily" or "slammed the door hard" because the verb "slam" implies that the action is forceful and also that the person might be angry.
Describing a sad character as "descending into a deep, dark, hollow pit of emptiness where everything is as black as night" is overkill. A better description might be, "Descending into darkness" or mention them having "a hollowness" inside.