Let's get one thing straight: dialog is not the same thing as conversation in the real world. There are some important similarities:
1) How someone says something is more important than the words themselves. Imagine your partner, a child, a politician, and a used car salesman (oops, I repeated myself!) all saying the words "I'm sorry." (Obviously, this isn't realistic in the last case. Just try.) Very different meanings, aren't they? This is why the little details of behavior--the smirk, the drumming fingers, the averted eyes are so important. We need to know those nonverbal cues.
2) The speaker determines the meaning of the words as much as the words determine the character of the speaker. In other words, when you first present a character, use the dialog to show their character. Later in the book, how the reader feels about the character will determine the meaning of the words. I know that's true for me in real life. Don't you know someone who if they say "Nice day." to you, you assume they're up to something crooked? And if your sweetie says, "Nice day," well, I blush to even consider what you might think!
There are also some big differences between dialog and conversation:
1) Someone is always listening: the reader. People are, by and large, lousy listeners. We're always thinking about what we're going to say next, what we need to get done or what the other person thinks about us. Readers are waiting for every word. You can use this to your advantage. Characters, because they resemble real people, do not always listen to each other, but the reader always listens. You can put in subtle bits of information that a character might miss for the reader's benefit.
2) Dialog is not just characters talking. It is the writer revealing relationships and information to the reader. While you write, your characters will lapse into small talk. That's natural. But you must prune it down like a topiary. Every word has to have a purpose.
