Once upon a time
I wrote a _____ rhyme
It wasn't very long
It was just a silly song.
Whatever word fits in the blank needs to have two syllables, first one accented, to fit the meter. If I wanted to emphasize the size of the rhyme, both small and little could have the right meaning, but only little fits the meter.
Verse (other than free verse--not going there now) imposes a form, and thus constrains word choice. The right word has not only the right meaning, but the right number of syllables and the right placement of accent.
In prose writing, other factors than meter constrain word choice (though meter is not unimportant even in prose--it's just less obvious.) Some words may not suit the intended audience, being either too formal or too informal. Some words may be forbidden in a particular publication or project. An editor or copy-editor may question a particular word: the overuse of gender-specific language in the past has led some to question or forbid its reasonable use now. In one of my books, a copy-editor suggested that weapons on a warship be "staffed" instead of "manned."
All writing intended for publication has an expected readership--and their reaction to the words chosen is part (but only part) of the internal sort that runs through a writer's mind when writing. This does not mean that writing needs to be dumbed down for some people, while others are smart enough to deserve the writer's best. Far from it, in fact. Most readers can comprehend unfamiliar words if the context is clear enough and the phrases in which the word is used are structured to make understanding easier. A writer skilled enough, elegant enough, can lift a reader's vocabulary in giant steps, without the reader feeling the strain. This is why I don't like restricted word lists, even for books aimed at younger children. It isn't the new words that give children problems with more advanced books--it's the concepts and the more complicated relationships between them.
But--saving that for a discussion at a later date on the writing itself--back to word choice and constraints. Another important constraint on word choice is tone. Tone is exactly what it sounds like: the "tone" of the "voice" in which you write. Some writing splutters with rage. Some writing chirps...some writing coos...some writing snickers. The tone of your writing determines, in part, how it's received--especially by those who don't like the message. How do you want to be "heard?" What tones are appropriate for the readership you're after?
When tone enters your mind, some words fall out of contention. If you want to convey anger, you'll leave out words that sound "soft" even if they have the right meaning, and any word that might hint that your anger is pretense--that you're being funny. You'll choose words that have strong, assertive associational meanings and a metric structure that fits an angry expression. Conversely, if you're trying to convey a desire for peaceful resolution, your word choices will shift to softer sounds, neutral to pleasant associational meanings, and a metric structure that fits a calm and friendly expression.
It was a mistake to think...
It was a blunder to think...
It was an error to think...
Error is the most neutral of the three; blunder is the most negative, the most emotive. Why? Error has scientific associations that tone down its emotional impact; blunder is more active (noun form of a verb) but rhymes with thunder...a blunder is a LOUD error. .and carries the associational meaning of clumsy (something no one wants to be.)
If you're writing a political piece intended to shock and arouse readers on your side, the other guy made a blunder. If you're trying to minimize the stupidity of your candidate, the same thing is an error.
In fiction, two constraints run side by side--characterization (characters think and act in words appropriate to them) and plot (plots have their own rhythm of tension and relaxation.)
Within characterization, dialogue most obviously defines character, so your word choice for a character's speech is critical: it must be true to the character's social status, cultural background, personality, as well as to the situation that made the character speak. Would your character call the person in authority in his/her job the boss, the manager, or the supervisor? Would your character, angry at someone, call that person a pusillanimous little pipsqueak or a cowardly dog? Characters' actions also define them, so accurate descriptions of those actions also require thoughtful word choices. Does the sullen petty criminal slump or slouch in the chair during interrogation?
Within plot, your word choice must enhance setting, atmosphere, rhythm, and pace. I once read a contest entry in which the writer tried to evoke a mysterious but beautiful forest scene inhabited by an equally mysterious and beautiful wood-nymph-like woman. One word destroyed it all...he described her beautiful green hair flowing down her back...like slime. Suddenly the mysterious forest and magical nymph were reduced to the status of a scummy pond. Now if you're trying to be funny, an obviously "wrong" word can be the right word...but this writer wasn't. To help reinforce the rhythm of the plot, and its pace (not the same thing--there are fast rhythms and slow ones) you need to know which words heighten tension and which relax it. Sometimes the same word, in a different sequence, will do the opposite--it takes experimentation and experience to learn when a word will perform for you. (This is why you need help from early readers of your stuff...) I've read passages that were supposed to be scary, but weren't--they were like a haunted house set by daylight, with all the curtains open and lights on.
What this boils down to is that of all the words you know--however many that is--only some of them will be eligible for any given piece of writing.