Let's talk about words. Without words, we have no writing....writing starts with putting one word at a time on paper or the computer screen. When I was young, I heard a lot of things about words and word choice in writing, most of them (on reflection) silly. "Use only short, strong, Anglo-Saxon words." "Never use foreign words." "Never use long words because most people don't understand them."
Poppycock (Look it up. Why should I spoil your fun?) The only advice writers need is "Use the right word." What is the right word? Ah...there's the mystery. But there are strategies for figuring it out. First, writers benefit from having a large...no, a huge vocabulary, especially of nouns and verbs. If you don't have the right word in your vocabulary, you can't use it, and you'll fumble around with substitutes, like someone trying to turn a screw with a thumbnail. The more things you know the right name of, the more things you can name in one quick word. Your animals will have their correct body parts, not "that place on the side where the ribs end..." The more actions you have the right verb for, the more compactly you can describe what the things and people are doing. You need fewer adjectives and adverbs piled on if you have plenty of nouns and verbs to play with.
Research has shown (at least, what I read on the internet about building vocabulary; they said it was research) that reading a lot is the best way to build vocabulary. Even better than conversation, once you're past the "Please pass the fish" and "I need a doctor; my head hurts" stage. Vocabulary assignments, vocabulary games and quizzes, have a good effect only if you subsequently use the word. So acquire (however you do it) as large a vocabulary as you can...and then use it only as needed, one word at a time. How big? Difficult to say, since the experts (the online experts anyway) can't seem to agree on what counts as a word (specifically, are closely related words off the same root really different words or not? Is "went" a different word from "go" or "sleeping" a different word from "sleep" or "done" from "undone?") If you're generous about word counting (yes, those are different words) then aim for something over 50,000. If you're parsimonious, at least aim for 30,000. (And where am I? Higher than that, but I'm not going to specify--for one thing, I'm not sure the methods of estimating vocabulary are all that accurate and the results I was getting seem unreasonably high.)
"Never" and "always" are dangerous directions for writers, because we usually rebel against them and go to the other extreme. So very gently I will say that finding the right word does not often mean looking for the impressive word. (Occasionally, it does. If someone is being an arrogant SOB and treating you like a peasant, smacking them in the face with a wet cold professionally competent term can have a good effect. And won't get you arrested for assault, which smacking them with a wet cold fish might. Just be sure you know how to pronounce it.) The purpose of writing is to get something across to your reader--the gist of the story you're telling, the real sequence of events at the school board meeting, the reasons why you're returning the shoddy merchandise and want a full refund.
Quite often the right word is an ordinary one, something everyone knows. Sometimes it's a less familiar but very specific word (the only right word for that object or action.) Sometimes it's a word few people know--but still the only word possible in that situation. (People who write SF and fantasy make up new words--words whose meaning no one knows but the writer when the story starts, and they have to make those words understandable quickly if they want to keep their readers.) But whether familiar, slightly familiar, or totally strange, writers must attend to putting the right word in each place. That matters far more than the length of the word or its familiarity.
That brings us back to the big problem...how to know which word is the right one. How did Shakespeare know when to use "incarnadine" (not a common word even back then) and when to use "redden?" How did Tolkein know that Galadriel should say she would "diminish" rather than "lessen" or "shrink?" How can you choose between "conceal" and "hide," between "grow" and "swell" and "enlarge?"
This is where the art of writing goes beyond the dictionary and the thesaurus, though both those resources are necessary companions to your lifelong study of words. Every word has the meanings listed in dictionaries...and additional cultural/social/emotional meanings accreted to it since the dictionary was published....and a physical sound when spoken...and all those affect its "fit" in the place you want to use it. It fits (or doesn't fit) not just because of its meaning (its dictionary meaning and its attached meanings) but also because of how it sounds in relation to the other words you put around it. A word in a phrase or sentence is like a puzzle piece in a puzzle--it doesn't matter if it's the right color (meaning) if the shape (sound) is wrong.
Vowel sounds color a word. Consider "uh" : the easiest vowel to say if you're recovering from a blow to the head--neutral, takes little effort. Now look at the "uh" monosyllables. Dull, dumb, slump, dump, bump, bug, lug, lump, pump, puff, huff, etc. Yes, there are "uh" words with a brighter feel ("hug") but on the whole, the "uh" monosyllables are have a gray tone. Consider "slump" and "slouch"...both describing "relaxed" posture, but in one case the result of depression, fatigue, misery...and the other a deliberate choice, usually in defiance. Within a phrase or sentence, vowel repetition--assonance--builds a stronger tone: Chesterton's "Strong gongs groaning..." or (just now made up) "silver is lily in pitcher..." (not much sense, but assonance all the same.) Too much assonance calls attention to itself--sometimes good, sometimes not--depends on your purpose--but a little assonance adds emphasis.
Multi-syllable words all have a pattern of emphasis (where the accent is) as well as a succession of sounds...and either join with others to make a rhythmic pattern or are canceled by others to deny a rhythmic pattern. In American English, the standard calm speaking rhythm is iambic...da-DA-da-DA-da-DA with small variations (the added unaccented syllable, often half-swalled): "He WANTS to GO to the STORE. " A reversal within any pair of syllables indicates intentional emphasis, pointing out a change from the expected: "HE wants to go to THE store." A reversal throughout (or almost throughout) defines whining: "YOU don't LOVE me" For the writer's purposes, the choice of a word with the right pattern of emphasis (when two words are close enough in meaning to be substituted) allows subtle control of the rhythmic pattern of the whole, and thus control of the tone. Long words make enforcing a rhythm easier ("and all the multitudinous seas incarnadine...")
Do you really need to worry about all that? Isn't the meaning enough? That's like asking if a musician needs to be aware of anything but the right pitch. (The answer, for non-musicians, is "no." Singing or playing the right pitch is just the beginning of making music.) The need for more than just the right meaning is clear in poetry but it exists in prose as well. Effective writing--in any form--requires a sensitivity to tones, rhythms, colors in the words you string together. Any writer may spend an hour or day picking up words like a jeweler picking up pearls to find the exact match for a place in a sentence. (You can't do that all the time or you'd never finish a short email, let alone a novel...but there are times when it's necessary and I've spent my hours sitting there picking at the dictionary, looking for the one right word for a given setting.)
And now my headache is gone (at least for awhile) so I'm off to bed, while blaming any incoherence in the above commentary on the departed headache, which can't defend itself.