• 'Mary had a dog' is a fact, not someone's opinion. I don't need to show everybody that she has a dog as it is not that important.
Of course it’s important. You say nothing that doesn’t develop character, set the scene, or move the plot (hopefully more than one of these at a time). Given that, the fact that you specifically mention the dog says the reader needs to remember it for later.
On the other hand, if you simply want her to have a dog to show her character, you have her pet, or feed, or even shout at the dog while she’s doing or talking about something else. That way the dog becomes enrichment for a necessary line. Showing doesn't mean the author describing something in the scene. It means placing the reader on the scene as the protagonist. We need to show them life, not vacation photos.
• Dennis was the most attractive boy in the school
Stop and think about that. In whose opinion? Surely you can’t believe that every single girl in the school thought him that? But that’s what you told the reader. And of more importance, you said it, not the character who thinks he's handsome.
If the character in your story says Dennis is the best looking boy that’s an emotional judgment, and meaningful to that character and the story. If you say it, it’s no more emotionally meaningful than learning the color of the classroom.
• Show takes up more time.
No, it doesn’t, I’m afraid. In fact, it takes a lot less, because the reader is deducing rather than being informed. If I say, “An errant gust of wind exposed skinny goose-pimpled thighs,” I’ve told you the weather is chilly, that the character is probably wearing a lightweight, and fairly short skirt, that she’s not dressed for the chill, that she has skinny legs, and that the rest of her is probably the same. In using skinny as against slim, or thin, I set a mode for her character, making her less desirable. By mentioning unattractive thighs rather than talking about her arms, I’ve made her sexually unattractive. And all that took only ten words.
• This would be show:
Mary played her music full-blast at 3AM. She butted in on conversations and frequently swore.
Sorry, again, but that’s the author, talking to the reader, directly, reciting facts, not involving the reader, emotionally. That’s telling.
Here’s one sample of the difference between show and tell. Which would you choose?
1. Fact-based:
John was sitting on the couch when a bullet came through the glass, narrowly missing his head. This was the third attempt on his life in the past week.
He threw himself to the floor and crawled toward the hall, and safety. Once there he pulled his pistol from its holster and went in search of his attacker.
2. Emotion-based:
The window by John's head exploded into shards, sending him diving to the floor.
Shit... again? Three times you try to kill me? Deciding that the man had persistence but lousy aim, John hurried to the hall, wincing as the shattered glass cut his knees and palms. Someone was going to pay for that with pain of their own.
Coming to his feet he pulled Beatrice from her holster and headed for the back door, releasing the safety. Okay, you bastard, you finally have me angry. No more "turn the other cheek" from me. [/I]
A thing or two to note:
• I didn't mention the window being broken by a bullet, the reader deduces that (or assumes it’s anything they care to, an option which can make it more meaningful to the reader), and in doing so becomes a participant.
• I didn't tell the reader he crawled, they deduce that by the damage he suffered.
• Each thing the character does is in response to a real-world stimulus, and in acting on it he influences the next stimulus he will act on. This is what makes the scene-clock tick forward.
• I deliberately gave the pistol a name because both the fact of it and the name speak of his personality. Had he been different I might have spoken of the caliber of the pistol, or something else that demonstrated that personality. His thoughts, too, fit the kind of person who names his pistol Beatrice. Put together, not only do we learn of the event, we learn of him, and his emotional response. Is there an entire history related to the shot being fired? Sure, but the action is what matters. Stopping that to have me come on stage and say, “Let me tell you a little about the character and what went before,” would kill any excitement I may have generated with the shot.