roadrunner1896 asks: What is the difference between "fast" and "quick"?
"Fast" and "quick" can be adjectives, adverbs, or nouns. In struggling to come up with examples, I think the words are most interchangeable when used as adjectives generally describing speed, but are less interchangeable (or not at all interchangeable) when used in adjectival phrases. (When the words are used as nouns they have completely different meanings and as an adverb "quick" should be "quickly.")
In their adjective forms, "fast" and "quick" are fairly similar as evidenced by 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 Roadrunner is fast.
The Roadrunner is quick.
Closing the Hellmouth (one of a zillion times) required fast thinking.
Closing the Hellmouth (one of a zillion times) required quick thinking.
However, reading these sentence pairs, one sounds "more right" than the other, doesn't it? Taking the literal meaning, all four examples are correct, however I'd be more likely to say "The Roadrunner is fast" and "Closing the Hellmouth (one of a zillion times) required quick thinking." Just a personal preference or is there something more there?
While still being used as adjectives, "fast" and "quick" can have very specific meanings when modifying certain words.
fast adj. characterized by unrestrained conduct or lack of moral conventions, esp. in sexual relations; wanton; loose
ex. Vala is fast, as evidenced by her penchant for leather.
fast adj. firm in adherence; loyal; devoted
ex. Harry and Ron were fast friends which is why Ron always wanted to come back.
quick adj. easily provoked or excited
ex. Touch Daniel's artifacts and you'll find out that he can be quick tempered.
quick adj. prompt to understand, learn, etc.; of ready intelligence:
ex. Hermione is a quick student, the cleverest witch of her age.
"Fast" and "quick" aren't interchangeable in those examples or for those meanings. (Though I think some people will mistakenly use "quick" in that first example of "Vala is fast".) And before anyone points it out: "a quick student" is different from "a fast learner." "A quick student" is implying something about the student herself, whereas "a fast learner" describes the speed at which one learns.
My fellow grammarian superhero_specs did further research on the etymologies of the words and came up with the following information:
Interestingly, it appears that fast "moving or capable of moving at a high speed" comes from the same root as fast "firmly fixed or attached" by way of "firmly, securely" -> "close, immediate" -> "immediately" -> "quickly".
Likewise, quick "moving fast or doing something in a short time" and quick "the soft, tender flesh below the growing part of a fingernail or toenail," both come from the same word meaning "alive, animated, alert."
Similar etymological information can be found at etymonline.com: fast and quick
Knowing those roots, it seems to imply that "fast" should primarily be used in conjunction with an action that occurs in a short period of time whereas "quick" has to do with the completion of an action in a short time (a "fixed" state (or speed) as opposed to an "animated" state).
I think you can see that in (some of) the examples used for the adjectival phrases:
Harry and Ron were fast friends which is why Ron always wanted to come back.
Harry and Ron continue being friends, a "fixed" condition.
Touch Daniel's artifacts and you'll find out that he can be quick tempered.
The action here of Daniel having a quick temper is instantaneous, an "animation" of anger.
It's interesting to look at those first examples—the ones in which fast and quick were interchangeable—with this etymological evidence in mind. What I thought was merely a personal preference may actually be based in usage. "The Roadrunner is fast" implies a continued state (the Roadrunner is always fast), whereas "Closing the Hellmouth (one of a zillion times) required quick thinking" is describing a single event.
Summary: So, roadrunner1896, to answer your question, the difference between "fast" and "quick" (when used as adjectives) isn't articulated in any solid-and-steadfast rules. It depends on usage and knowing that while the words are synonyms, they can't arbitrarily replace each other. Looking at what is being described and whether or not it is in a "fixed" or "animated" state may be the best way to handle this particular facet of the English language.
All definitions from dictionary.com