Which character came to life in an especially vivid way? How did the writer achieve this effect?
This is a question that is answered by interpretation from the reader, but (to me, as a reader) the old man was the character that was described with more vivid detail, regardless of the fact that the narrator himself was the one with most of the action falling onto himself, and his movements having been described so meticulously.
The old man, however, had the eye that bothered the narrator so much. It is the vivid detail with which the eye is described that makes one think that one can almost see it, touch it, and become repulsed by it. The writer achieved this effect with very palpable emotional descriptors:
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! One of his eyes resembled that of a vulture -- a pale blue eye with a film over it. Whenever it fell upon me my blood ran cold, and so by degrees, very gradually, I made up my mind to take the life of the old man, and thus rid myself of the eye for ever.
I agree with the previous post that the old man is the most vividly described character in "The Tell-Tale Heart." The narrator's declaration of the "vulture eye" in itself creates an image that is unforgettable. There is little other specific information that describes the victim, but it is easy enough to create a mental image of the terrified old man in his nightgown as he listens for the sound that he believes to have heard; and to visualize him as he squints uncertainly to see the tiny speck of light that he believes to have seen.