It is obvious enough that Plato's literary style, including his use of dramatic form and character, alters drastically along with his philosophical method. It is most economical, though not essential, to attribute these parallel changes to Plato's own chronological development. As Guthrie puts it, "Plato began by giving vivid pictures of Socrates engaged on his mission, and as he went on became more concerned to develop positive doctrines. He retains the dialogue form, but it becomes less dramatic and pictorial and he allows Socrates to indulge in uncharacteristically long discourses only punctuated by expressions of assent from the others" (HGP iv.42). But why? Why does Plato use dramatic form to portray Socratic inquiry in the early dialogues? And why does he employ that form so differently when representing more "positive" ideas?
The Republic is a unique crucible for examining such questions, because of the marked alteration in literary and philosophical style after the first book. Book 1 resembles the "early" or "Socratic" or "elenctic" dialogues, and as such deploys dramatic form and character very differently from the remainder of the work. Since Books 2-10 are clearly intended as a continuation of Book 1 (whatever their relative dates), we may expect the stylistic shifts to tell us something about Plato's own shifting attitudes towards philosophical method and its literary expression. I shall therefore start by looking at the characters of Socrates' interlocutors in book 1 of the Republic, and trying to elucidate the relationship of characterization to the method that is portrayed there. I'll then turn to the radically different dramatic style of the rest of the work, and end by considering some possible reasons for this evolution.
Blundell, Mary Whitlock and Blondell, Ruby, "Character and Method in Plato's Republic" (1992). The Society for Ancient Greek Philosophy Newsletter. 323.