“Dialogue is among the most misunderstood of writing tools. One misconception has to do with dialogue’s function in the story: most writers ask their dialogue to do the heavy lifting, the work that the story structure should do. The result is dialogue that sounds stilted, forced, and phony. But the most dangerous misconception about dialogue is the reverse of asking it to do too much; it is the mistaken belief that good dialogue is real talk. Dialogue is not real talk; it is highly selective language that sounds like it could be real. Good dialogue is always more intelligent, wittier, more metaphorical, and better argued than in real life. Even the least intelligent or uneducated character speaks at the highest level at which that person is capable. Even when a character is wrong, he is wrong more eloquently than in real life.”
—John Truby, The Anatomy of Story, 2007, pp. 376–7 (emphasis in original)
PS See also TV Tropes, “Realistic Diction Is Unrealistic”, tvtropes.org/pmwiki/pmwiki.php/Main/RealisticDictionIsUnrealistic.