Show Vs. Tell, as I understand is, comes more into play in the external.
John was angry about the mess in the room. "This really pisses me off," he said.
versus...
John's brows collided in a red twist. He shoved his hands at the piles of dirty clothes. "This really pisses me off." -Jason
I heard a writer once say that the narrator is a liar, only the characters speak the truth. I think this is a good rule to follow when determining how to show, not tell. So what does it mean? First, it does not apply to first person writing because with that style, the narrator is a character, usually the main character. But in the case of third person, the only thing the reader can really believe is what is revealed through the words, actions and reactions of the characters. If the narrator tells the reader something, we must assume it is a lie because the narrator is not a character and is therefore “outside” of the story--what Jason calls external. Even if a character tells a lie, the reader can at least believe it’s a lie. But the narrator has no credibility, therefore cannot be believed. If possible, all information should come through the words, actions and reactions of the characters to be the truth.
So the characters “show” us while the narrator “tells” us. In the case of Jason’s example above, we are told that John is angry. Who’s opinion is that? It’s the narrator’s opinion. We are being “told” by the narrator that John is angry. It might be true, but it could be a lie.
In the second example, there’s no doubt that John is angry. His anger is shown through his actions using action verbs: collided, shoved. And his anger is supported by his words. The reader can believe the character in the second example is angry, but not necessarily in the first.
So as you write, remember that the narrator is a liar, only the characters speak the truth. If you reveal information through the words, actions and reactions of your characters, there will never be an issue of credibility, and no one will accuse you of telling, not showing. Good luck.