I realize that Stephen's answer is pretty thorough, but can't help but think that side-by-side comparison of their definitions may help a bit, so...
Question: Noun: A sentence worded or expressed so as to elicit information.
and
Invitation: A written or verbal request inviting someone to go somewhere or to do something.
So the answer to your question "Do they accomplish different things?" is "Yes".
They do. question is for obtaining information, and invitation is more for giving information.
From New Oxford American:
dark horse |ˈdɑrk ˈˈhɔrs|
noun
1 a person about whom little is known, esp. someone whose abilities and potential for success are concealed : [as adj.] a dark-horse candidate.
2 a competitor or candidate who has little chance of winning, or who wins against expectations : a preseason dark horse as the nation's top collegiate football team.
sleeper |ˈslipər|
noun
3 a movie, book, play, etc., that achieves sudden unexpected success after initially attracting little attention, typically one that proves popular without much promotion or expenditure.
• an antique whose true value goes unrecognized for some time.
From Etymonline:
dark horse
in politics, 1842, an image from horse racing, in which dark is used in its figurative sense of "unknown."
Moonraker is called a "dark horse"; that is neither his sire nor dam is known. ["Pierce Egan's Book of Sports," London, 1832]
sleeper
[...] Sense of "something whose importance proves to be greater than expected" first attested 1892, originally in Amer.Eng,. sports jargon, probably from earlier gambling slang (1856) sense of "unexpected winning card."
Here's some generalizations gleaned from the above:
- dark horses are usually people; sleepers are usually things.
- dark horses are intentionally kept unknown; sleepers are simply not promoted.
- a dark horse might achieve success; a sleeper already has—surprisingly.
- dark horses' limelight often decrescendoes; sleepers' crescendoes.
As to your questions, I'd say they're not quite interchangeable. Knowing their different connotations will likely prove one to be the better word choice.
A dark horse might also be called an enigma, or simply an unknown; while surprise hit, cult classic, and box-office success are other phrases often applied to sleepers.
Best Answer
To "have mercy" is the plea of someone who desires mercy before it has been extended. The publican, or tax collector, who stood at a distance from the Jewish temple in Jesus' day, not so much as looking up to heaven "beat his breast and said, 'God, have mercy on me, a sinner'" (Luke 18:13 NIV). In this context, Jesus contrasted the words of a self-righteous Pharisee ("'God, I thank you that I am not like . . . this tax collector. I fast twice a week; I pay tithes of all that I get'") with those of the tax collector. He said, "'. . . this [tax collector] went to his house justified rather than the [Pharisee}, for everyone who exalts himself will be humbled, but he who humbles himself will be exalted" (ib., v.14).
In order to receive mercy, one must humble himself, thus acknowledging tacitly there is a reason why mercy should not be extended. If the person from whom one is requesting mercy is merciful, then mercy will likely be extended. There is a pithy statement that goes: "Grace is getting what you don't deserve; mercy is not getting what you do deserve."
In confining my search for mercy strictly to the Bible, I found the following expressions. In no particular order: "show mercy," "without mercy," "receive mercy," "great mercy," "full of mercy," "rich in mercy," "had mercy," "desire mercy," "love mercy," "withhold mercy," "with mercy," "request mercy," "cry for mercy," "lift up my voice for mercy," "gets no mercy," and "beg for mercy."
When someone asks another person to "have mercy," the implication is that a punishment, though deserved, can also be withheld, if the one who can extend mercy chooses to.
I remember seeing a movie--a "western"--years ago, in which a bad guy begs the protagonist (Clint Eastwood?) for mercy. The protagonist says wryly, "How is it that those who ask for mercy never give it?"
That reminds me of the ancient story of a debtor and his creditor. The creditor to whom the debtor owed a huge sum showed him mercy and forgave the debt. What did the ecstatic man who had been shown mercy do? He turned around and withheld mercy from one of his own debtors, who owed him a piddling sum. He then had him put in debtors' prison until the last penny was repaid. Ironic, isn't it?