If the dialog in a movie doesn't ring true, it can affect the way you feel about the whole story. Dialog is any conversation in a book, film or play.
Dialog is more commonly spelled dialogue, but either way it means "lines spoken by a character" or a conversation, either between fictional characters or real people. You might have a dialog with your boss if you're unhappy with your work hours and want to sit down and discuss your schedule. Dialog comes straight from the Greek dialogos with pretty much the same meaning, although it was originally used to mean a very formal type of speech or conversation.