Sure did! Plato uses historical people as interlocutors in his dialogues, and frequently has the same people appear in multiple dialogues (Euthyphro appears in both the eponymous Euthyphro and in the Cratylus). It’s important to keep in mind here that Plato’s dialogues are set in a time in Plato’s past, making them a kind of historical fiction for his own time, and they frequently featured important and well-known philosophers, sophists, playwrights, and historical actors. Even today, most people who pick up the Symposium probably already know who Aristophanes is (yes, that Aristophanes, paired here with the — slightly less well known in the modern world — tragic playwright Agathon), and a character like Alcibiades (who makes an appearance, even if he doesn’t always speak, in quite a few of Plato’s works, even disregarding the fairly certainly not-Plato Second Alcibiades and the probably not-Plato Alcibiades, both attributed to Plato) was more than just mildly infamous.

The histories and characters of the people who populate Plato’s works are an important — and frequently neglected, when quoted out of context — part of the dialogues and how they construct meaning, and the fact that Plato used real people for this gives an extra dimension to the question. It gives a whole different charge to the ending of the Charmides, for instance, that the conversation takes place between Socrates, a still-very-young Charmides (Plato’s uncle), and Critias (Charmides’ and Plato’s mother’s cousin), the last two of whom were infamous for their involvement with the Thirty Tyrants (yes, that Critias) about sophrosune, frequently translated as “temperance”, and ends with Critias commanding Charmides to learn temperance from Socrates — by force, if necessary. Its final lines are:

“What use is counsel?” said I [Socrates]. “Because when you undertake to do anything by force, no man living can oppose you.”

“Well then,” he said, “don’t oppose me.”

“Very well, I shan’t,” said I. (Pl. Chrm. 176d, translation from Sprague)

These lines about use of force mean something different coming out of the mouth of Charmides, whose future was well known, even if the Charmides of the dialogue isn't aware of it yet, than from some random made up character of no historical significance, and the dialogues make frequent use of this kind of future knowledge on the part of their audience. While we know less about Euthyphro in particular than we do about many of Plato’s other interlocutors, most of those who appear in Plato are also attested in other sources, so we know more about them than appears in the dialogues themselves, and who they are is important for the dialogues themselves.

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Nails, Debra. The People of Plato: A Prosopography of Plato and the Other Socratics. Hackett, 2002.

Sprague, Rosamund Kent. “Charmides.” In Plato: Complete Works. Edited by John M. Cooper and D.S. Hutchinson. Hackett, 1997.