I was reading a game of thrones and got an ambiguous understanding of two words: fast and quick It looks like the author is describing two characters whose characteristics are quite opposite. Is there any subtle difference?
It is difficult to differentiate these two words. I think it helps a bit to say "fast" refers to movement, "quick" to reaction. But a lot of uses are idiomatic and a matter of the dictionary. A clean differentiation of "fast" and "quick" is not possible as there is an overlapping area. The matter is complicated as "fast" has two meanings, it can refer to speed and it can express firmness. Dictionaries should have two entries, but often "fast 1" and "fast 2" are intermingled in one dictionary entry. Normally dictionaries are not very good at differentiating words whose meanings are very close. If you compare dictionary explanations of "fast" and "quick" you often find such definitions/explanations don't help at all. Often dictionaries with up-to-date lexicology try to solve such problems by giving a special information box where more information about the use of words with similar meanings is given. But I haven't seen such an information box for "fast" and "quick" yet.
In many cases the two words mean the same thing.
If you are encouraging people to hurry you may urge them on with either Do it quick! or Quick! or Do it fast! or Fast!
You may describe a good student as a quick learner or a fast learner.
But in other contexts they may have somewhat different meanings.
Fast tends to be used when you are speaking of the time it takes someone of something to move a given distance: a fast train, or a fast runner.
Quick tends to be used when you are speaking of how long it takes someone to respond to something: a quick answer to a question, or events following each other in quick succession.
The distinction is particularly marked in the way we speak about athletes. A batter in baseball, or a batsman in cricket, must have a quick eye, to react in a fraction of a second to the pitched or bowled ball. But when he runs he must be fast, to cover the distance between bases or wickets in as short a time as possible.
"Fast" and "quick". I think that these words are interchangeable when used as adjectives generally describing speed, but are less interchangeable ,or not at all interchangeable when used in adjectival phrases.
Adjectives In their adjective forms, "fast" and "quick" are fairly similar ; look at their definitions.
fast adj. moving or able to move, operate, function, or take effect quickly; quick; swift; rapid quick adj. done, proceeding, or occurring with promptness or rapidity, as an action, process
You can say: The train is fast. The train is quick.
Adjectival Phrases While still being used as adjectives, "fast" and "quick" can have specific meanings when modifying certain words.
fast adj. characterized by unrestrained conduct or lack of moral conventions, esp. in sexual relations; wanton; loose ex. Nancy is fast, as evidenced by her way of speaking and her cheap clothes. fast adj. firm in adherence; loyal; devoted ex. Joe and Chris were fast friends until the day they died.
quick adj. easily provoked or excited ex. Your father has always been quick tempered. quick adj. prompt to understand, learn, etc.; of ready intelligence: ex. George is a quick student.
They have the same meaning. "Quick" or "Quickly" is slightly more formal. "Fast" is more likely to be encountered in casual conversation.
When I was teaching English in Korea, they had taught all of the kids to say "quick" and it sounded odd to me. "Fast" is more likely to be encountered in everyday conversation.