From Simple & Direct: A Rhetoric for Writers, by Jacques Barzun:
The whole world will tell you, if you care to ask, that your words should be simple & direct. Everybody likes the other fellow’s prose plain. It has even been said that we should write as we speak. That is absurd, as we know from the courtroom dialogue. Most speaking is not plain or direct, but vague, clumsy, confused, and wordy. This last fault appears in every transcript from a taped conversation, which is why we say “reduce to writing.” What is meant by the advice to write as we speak is to write as we might speak if we spoke extremely well.