To flesh out is to give substance to something. The idiom flush out (originally from hunting) means to bring something out in the open.
Examples
Flesh out
President Barack Obama will today attempt to flesh out his energy strategy with the unveiling of a major new incentive scheme designed to improve the energy efficiency of commercial buildings. [Business Green]
The prequel, which aims to flesh out Sam Axe’s backstory, is set in late 2005. [TV Squad]
Some of these poets and communities were hard to find because they didn’t have fleshed out websites or buildings with addresses in the phone book. [Chicago Tribune]
Flush out
Omaha police are hoping for help from the public to flush out a thief who has been stealing brass from public restrooms. [KETV Omaha]
A drive in the forest to try and flush out “Bigfoot” will be held Saturday, Feb. 12. [Montgomery Herald (article now offline)]
Jynx flushed Connor out and bit him as he opened fire with an AK-47. [Reading Eagle]