Antonomasia
Using a proper noun as a common noun (a Scrooge) or a periphrasis in place of a proper noun.
Antonomasia moves a proper noun into common-noun territory — "a Scrooge" for a miser, "a Casanova" for a seducer — or, in reverse, replaces a proper noun with a characterizing periphrasis: "the Bard" for Shakespeare. The name becomes a type; the character becomes a category.
Some antonomasias entered the language so deeply they erased their origin (a sandwich, from the Earl of Sandwich). For the writer, a live antonomasia is a powerful characterization shortcut: saying a character thinks he's Napoleon summons an entire file in one name.
Example
"A Scrooge" (a miser); "the Bard" (Shakespeare).