The music of poetry is not like the music of an instrument, however. It is something different. Music in poetry is obtained by catching the conversational tones which are the special property of vital utterances. There is the sense the words convey, and there is also an emotional quality, an interpretative quality, in the tone in which the words are uttered. To gather these, because they are significant and vital and carry through the ear an appeal of sincerity, is a main effort in poetry.
Conversational tones are numerous in dramatic poetry.
As a result, the dramatic is the most intense of all kinds of poetry. It is the most surcharged with significance. Frost cannot write unless he can hear in him the voices carrying on the conversation that he records. These visitations of style, as Stevenson calls them, are not at his command. At other times he can't write at all.