The key to writing good dialogue is honesty

"As with all other aspects of fiction, the key to writing good dialogue is honesty."
"You must tell the truth if your dialogue is to have the resonance and realism that Hart's War, good story though it is, so sadly lacks"
"The point is to let each character speak freely, without regard to what the Legion of Decency or the Christian Ladies' Reading Circle may approve of. To do otherwise would be cowardly as well as dishonest, and believe me, writing fiction in America as we enter the twenty-first century is no job for intellectual cowards. There are lots of would-be censors out there, and although they may have different agendas, they all want basically the same thing: for you to see the world they see ... or to at least shut up about what you do see that's different."
(#2 p.184-185)