Lots of words sound so much alike (affect/effect) or are so nearly similar (historic/historical) that most of us have trouble sorting them out. Also, we may know that both "propensity" and "proclivity" have something to do with inclinations, whether by habit or by nature. But we may not know that "proclivity" is best used to describe a predisposition toward something unwholesome or undesirable, while a "propensity" is merely the general direction one tends to go.
There are lots of these confusible word pairs in English. Some are triplets -- preface/prelude/prologue; there are some quadruplets -- immigrant/emigrant/emigre'/migrant.
The best confusibles are made up of words that sound somewhat alike and live in the same semantic neighborhood. Thus, both "inept" and "inapt" have to do with the idea of inappropriateness or unsuitability, but an inept remark is a foolish one, while an inapt remark is merely unsuitable for the occasion. (When applied to people, "inept" means slow to learn while "inapt" means lacking in skill.)
Or, to take another example, in the confusible exceed/excel, both words carry the connotation of "surpassing." But "exceed" means to go beyond in terms of something measurable, like a quantity, extent, or time, as when we exceed the speed limit; "excel," on the other hand, has a qualitative sense about it, a surpassing in the sense of doing better in a comparative way (a runner excels by exceeding his or her best time for the mile).
A few more fairly common confusibles:
aggravate/exacerbate -- Both have a root meaning of "irritate," but "aggravate" means to make an irritation worse, as when we scratch a patch of poison ivy. "Exacerbate" carries a finer distinction -- to increase the severity of an irritation by making it more bitter or harsh.
allocate/allot -- To "allocate" something is to set it aside for a specific purpose. A common executive perk is an allocated parking space. To "allot" something is to dispense it at random (we all receive our lots in life), but often with some restriction attached to the gift, as when you are alloted speaking time at a meeting.
continually / continuously / constantly -- All three suggest repetition, but in different ways. Use "continual" to mean events that repeat regularly and frequently: "The night of the party, the doorbell rang continually." Use "continuous" to mean something that goes on without interruption: "Senator Phil E. Buster spoke continuously for thirty-three hours." Use "constant" when you want to suggest not only continual, but an unchangeable quality as well: "Basil's puns kept us groaning constantly."
disinterested/uninterested -- This pair is a near-perfect confusible. The two words are almost exactly the same, are almost universally confused, yet refer to two very different kinds of interest. "Disinterested" means without interest in the sense of having no stake in the matter at hand. Judges should be disinterested, i.e., impartial. "Uninterested" means lacking in interest -- being without any curiosity or fascination: "Druscilla is uninterested in Albanian folk songs."
insinuation/innuendo -- Both are indirect suggestions. An "innuendo" is a derogatory remark about someone that attacks his or her character: "Have you heard about the embezzlement over at Last National? And since when can Figby afford a Ferrari?" An "insinuation" is often negative as well, but is less forceful than an innuendo; it's more like an insightful hint: "Is that arched eyebrow an insinuation about my veracity or just a facial tic?"
licentious/lascivious/lecherous -- All three have to do with lewdness or lust. "Licentious" acts are characterized by license, i.e., they lack moral restraint. "Lascivious" means a predisposition toward lewd or lustful behavior. But "lecherous" means actually given over to grab and tickle and whatever else. Together, they create a direct progression: licentiousness gets rid of the inhibitions, lasciviousness provides the disposition to act, lecherousness is the nitty-gritty.
proceed/precede -- Both have the sense of moving ahead, especially after having stopped. But "precede" carries with it the additional connotation of already being ahead, in the sense of taking precedence, whether motion is involved or not. Queen Elizabeth and Prince Philip will illustrate the point nicely: When their royal highnesses proceed with any procession, she precedes him, and not just because it's ladies first.
tumult/turmoil -- Each is a kind of disturbance, but a "tumult" is a "turmoil" (state of agitation or disorder) accompanied by noise. A "tumult" also implies a large number of people. The origin of "turmoil" is unknown, but the verb "moil" (to toil or slave) was put to wonderful use by Robert Service in "The Cremation of Sam McGee": "There are strange things done in the midnight sun/By the men who moil for gold." A lot of tumult accompanied the turmoil in the Yukon, which was the setting for the poem.
venial/venal -- "Venial" applies to sins that are relatively minor, that merit only temporal punishment, and that are easily forgiven; "venal" applies to character itself and means open to bribery.
Whether you find yourself amused or bemused by all this, you may still wonder what difference it all makes. Granted, some day, billions of years hence, the sun will go out and the Earth will be a cinder in the eye of the cosmos. When that happens, not too many people will care who is going into "hysterics" and who is going into "histrionics." But even then, some lonely soul may be around to point out that the hysterical ones are genuinely afflicted, while those indulging in histrionics are still keeping one eye peeled to calculate the impact of their theatrically tinged behavior. And I hope that soul, whoever it may be, will have occasion to recall Mark Twain's definitive statement on usage: "The difference between the right word and the almost right word is the difference between lightning and lightning bug."