Diction refers to the author's choice of vocabluary and style of expression in a story, poem, or any other piece of writing. In the case of Edgar Allan Poe in The Tell-Tale Heart, the chosen diction is that of a frantic, almost schizophrenic language, in which the speaker is desperately trying to convey to the reader that he cannot handle the prescence of the old man. Right away in the opening lines of the story, the speaker's fragmented speech gives away his inability to cope with reality as he falls over the word, "nervous." The speaker repeats some words, like "nervous" several times. Most sane people will simply tell the story as is, in some intelligable manner, but Poe's narrator is already out of touch with reality. This can be seen when the speaker insists that he can indeed hear voices and sounds of the beating heart that are not actually there , "I talked more quickly - more vehemently; but the noise steadily increased...but the noise steadily increased!" (Poe)
The policemen, of course, do not hear anything out of the norm, so this serves to exacerbate the speaker's paranoia and delusional state. Poe's use of language repetition, "I foamed - I raved- I swore...but the noise arose over all and continually increased. It grew louder, louder, louder!" (Poe), shows just how demented the speaker really is. He cannot fathom that any of this is happening only in his own twisted mind, and that no one else can hear anything. Whether it is borne of guilt for killing the old man or not, the speaker begins the story with similar language.
We can therefore make the supposition that the speaker is mentally imbalanced. Poe uses ominous words, repetition of words, and a frantic, almost stuttering pattern of speech in order to create horror for the reader. He assumes the reader to be an outsider who thinks reasonably, one who can differentiate fact from fiction or reality from delusion. This use of diction is necessary as Poe weaves a dark tale of madness and suspense. It is a constant sense of psychological build-up for the reader, as if we are traveling some twisted path inside the speaker's mind.