Emily Dickinson's word choice was a. always formal B. always informal c. a combination of formal and everyday dictation?
This is an interesting question when we consider the style of Dickinson in her excellent poetry. You might want to think about posting this question on the discussion board as you will get a wide range of views. My own personal response has to be C, as although B is correct on the whole, I don't think it is fair to limit her work to simple word choice as she does challenge and stretch us by the way she uses words.
Dickinson's language often suggests the spontaneity of speech. Many of the features of spoken language, which tends to be disjunctive, fragmented, elliptical, to contain more questions, exclamations and pauses than most written language, appear in her poems, which frequently begin with "I," another marker of speech. However, this sense of spontaneity must be balanced by the aphoristic style of her poetry, where the "I" is replaced by universals. There is a definite air of solidity and finality about these aphorisms which is undercut by the irregularities and disorder of the speech-like lines. You might want to consider the poem "He fumbles at your Soul" for an example of this in action.