"Yes, I finally have the mystical scroll that I need to destroy those pesky do-gooders!"

Who talks like that (except in Scooby-doo)? Stupid people make terrible leaders, and powerful leaders are very careful about what they say.

Dialog like this is the author's way of explaining a plot, nothing more. Perhaps it helps, but why explain the plot in this way? Plots can be shown using action much better, and the cost of trite dialog destroys the realism of the moment. If your characters don't speak like real people, who is going to believe in them?

In writing, distancing your reader, reminding them that they are only reading a story, takes them out of the story.

How often do you hear someone say this:

"Work is done; now I can go to the bathroom and not be hurried as I do what I must before I flush!"

If people don't talk like that in the real world, then why would characters? Believable dialog helps make characters believable.

Gloating is acceptable for two teenagers eating pizza and comparing their last romantic encounters (assuming that they've ever had any). Professional people don't gloat; why should overlords? Sauron raised the greatest army in Middle-Earth, but Tolkien didn't describe him trying to impress some random orc by bragging. Powerful lords care more about gaining power than diluting it by boasting about their evil triumphs to their enemies; if it doesn't increase their power, why would they do it?

Dialog by a hero is often equally bad:

"Oh, I must sacrifice myself and risk my virginity in the hope that everyone's lives will be spared from the doom of the Empress of Vaginalla."

(Give me a break, please!)

Personally, if I ever met any character so naive that the sight of a sexual organ makes them blush, then I would laugh at them. Even in my private conversations with God I would never speak like this. Noble people are often very good speakers, but true nobility is shown in people's actions, not through their words.

If you need dialog like this, to explain to the reader that the hero is about to sacrifice themself, then you need to rewrite the scene. If you think that dialog like this works, try using it on your current sexual partner during an argument:

"Obviously I am right, for all the signs of the universe point to my total and inescapable correctness in all matters, whereas your contradiction of my beliefs assures your incorrectness, and therefore you must submit to my reasoning."

See how far that gets you.

Dialog that is never used can be the most powerful. When a character wants to say something obvious, it is often more powerful to use internal dialog, to have them turn away and say nothing. All humans relate to this; we have all had to keep our mouths shut at one time or another when saying what we wanted to would only cause us trouble. Besides, why repeat the obvious? Redundancy is sometimes required, but it should be extremely sparse, used only when a point must be made for future reference.

Michael's lips parted, but he uttered no sound. He had wanted to break up with Janis, but he wanted to be the one to initiate it. He'd thought that she had eyes for no one but him; how could he have been so wrong? Michael gave Janis a dark, penetrating stare, an expression of his fury that he wanted to linger in her mind, and then he walked away without a word. He was glad to be rid of her; in private, he would give a cheer, but right now, all that he wanted was for her to see was his back.

This is internal dialog, and it is better than:

"I was going to break up with you anyway, you worthless whore."

Although shorter, the internal dialog makes Michael interesting and sympathetic, while the verbalized dialog makes him a jerk.

Timing is important for dialog. The worst examples of this are talking while running, fighting, or hiding. When evil men are searching for you, do you really think that they can't hear you whisper? When running for your life, do you usually have breath for conversation? When about to fight your mortal foe, do you often waste breath giving a speech about morality and the importance of being nice? Conversations happen at two times; when it must, and when the action pauses to allow it. Traveling from one scene/situation to another is a great time for conversations; in the middle of the scene/situation is often the worst time.

Conversers should seldom agree. Why discuss something that you both agree upon? Arguments are more fun and interesting; arguments draw stark contrasts between your characters and help to make each more unique. Use arguments frequently.

As writers, we choose the dialog that best promotes our stories, but it should always be a conscious choice. Characters should use dialog that fits their personality, the setting, the tone of the story, and the urgency of the moment. One trick is to go through your story once for each main character and read only their dialog aloud; does it sound consistent? Do they use complex words in one part but mostly-simple words in most? Could their voice (style of talking) be confused for another character? Is everything that they say from their perspective ... and convey their unique attitude?

How do you know if dialog is good or bad? In any situation, imagine your best friend shouting out a semi-appropriate line from your story; would you laugh at them?

Good dialog is essential to a good story. My most-hated dialog is:

"If I don't get out of here right now then I am going to die a horrible death!"

In this situation, I only use two words:

"Oh, fuck!"