Aristotle, and W. Rhys Roberts. Rhetoric. Mineola, NY: Dover, 2004. Kindle Electronic Edition.
Book III, chapter 7.
Aristotle discusses appropriate language in book three chapter seven. Your language will be appropriate if it expresses emotion and character, and if it corresponds to its subject” (Aristotle III.7, B. 1408a). To correspond to the subject, he says the language choices must match the subject in seriousness or levity. The result of appropriate language is an air of credibility. While the tone of communication should not always match the topic, thus seeming too much like a theatrical performance, it should generally be close, giving a genuine and unstudied air (Aristotle III.7, B. 1408b).