To flesh out something (like a plan or an idea) is to expand it or give it substance.
To flush out means to force someone or something out of hiding or to clean something (usually by forcing water through a container).
"If you are trying to develop something further, use flesh; but if you are trying to reveal something hitherto concealed, use flush."
(Paul Brians, Common Errors in English Usage. William, James & Co., 2003)
- The president promised to flesh out the details of his troop withdrawal plan.
- The tides coming from Nantucket Sound are not high and forceful enough to flush out the nitrogen, which causes algae and seaweed to flourish.
Practice:(a) Gus tried to _____ out his novel with incidents borrowed from other writers.
(b) An undercover operation may be the best way to _____ out would-be terrorists.