How does the character or setting show the theme of "insanity" in the story "The Tell-Tale Heart?"
Practically every word from the mouth of the character (speaker) tells us he is insane.
At the very beginning of the story, the words the speaker uses and the kinds of sentences he speaks hint at his insanity. His sentences are short and they sound very frantic.
I loved the old man. He had never wronged me. He had never given me insult. For his gold I had no desire. I think it was his eye! yes, it was this!
When you read that sentence, it is very easy to see in your mind's eye a man pacing up and down speaking very quickly and frantically.
Later in the story he also talks about feelings he has that make him seem insane. He talks about the fact that he is affected by some nameless terrors in the night.
I knew the sound well. Many a night, just at midnight, when all the world slept, it has welled up from my own bosom, deepening, with its dreadful echo, the terrors that distracted me.
So both the things the character says about himself, and the way he speaks them, make him seem insane.