Sometimes, dialogue reveals the cultural/ethnic background of characters, too. For instance, Alexander McCall Smith's dialogue is a perfect fit for the Botswana setting and cultural backgrounds of the characters of his No. 1 Ladies' Detective Agency series. Roderic Jeffries' Insepctor Alvarez series also has culturally authentic dialogue. Authentic dialogue can be very tricky, especially if the author isn't a member of the culture about which he or she is writing. If the author isn't very careful, dialogue spoken by members of other ethnic or language groups can sound patronizing (Thanks, Kerrie, for making me think about that!).
What do you think about dialogue? In your favorite series, how does the author use dialogue? Does the author give clues through the dialogue? Does dialogue in other languages (especially languages you don't speak) detract from a mystery?