I think that what fundamentally characterizes the prophet's truth-telling, his veridiction, is that the prophet's posture is one of mediation. The prophet, by definition, does not speak in his own name. He speaks for another voice; his mouth serves as intermediary for a voice which speaks from elsewhere. The prophet, usually, transmits the word of God. The discourse he articulates and utters is not his own. He addresses a truth to men which comes from elsewhere. The prophet's position is intermediary in another sense in that he is between the present and the future. The second characteristic of the prophet's intermediary position is that he reveals what time conceals from humans, what no human gaze could see and no human ear could hear without him. Prophetic truth-telling is also intermediary in that, in one way of course, the prophet reveals, shows, or sheds light on what is hidden from men, but in another way, or rather at the same time, he does not reveal without being obscure, and he does not disclose without enveloping what he says in the form of the riddle. Hence prophecy basically never gives any univocal and clear prescription. It does not bluntly speak the pure, transparent truth. Even when the prophet says what is to be done, one still has to ask oneself whether one has really understood, whether one may not still be blind; one still has to question, hesitate, and interpret.
Now parrhēsia contrasts with these difference characteristics of prophetic truth-telling in each of these precise respects. You can see then that the parrhesiast is the opposite of the prophet in that the prophet does not speak for himself, but in the name of someone else, and he articulates a voice which is not his own. In contrast, the parrhesiast, by definition, speaks in his own name. It is essential that he expresses his own opinion, thought, and conviction. He must put his name to his words; this is the price of his frankness. The prophet does not have to be frank, even when he tells the truth. Second, the parrhesiast does not foretell the future. He helps them in their blindness, but their blindness about what they are, about themselves, and and so not the blindness due to an ontological structure, but due to some moral fault, distraction, or lack of discipline, the consequence of inattention, laxity, or weakness. It is in this interplay between human beings and their blindness due to inattention, complacency, weakness, and moral distraction that the parrhesiast performs his role, which, as you can see, is consequently a revelatory role very different from that of the prophet, who stands at the point where human finitude and the structure of time are conjoined. Third, the parrhesiast, again by definition, and unlike the prophet, does not speak in riddles. On the contrary, he says things as clearly and directly as possible, without any disguise or rhetorical embellishment, so that his words may immediately be given their prescriptive value. The parrhesiast leaves nothing to interpretation. Certainly, he leaves something to be done: he leaves the person he addresses with the tough task of having the courage to accept this truth, to recognize it, and to make it a principle of conduct. He leaves this moral task, but, unlike the prophet, he does not leave the difficult duty of interpretation.
—Michel Foucault, The Courage of the Truth (The Government of Self and Others II): Lectures at the Collège de France, 1983-84 (pp. 15-16 in ISBN 978-1-4039-8668-9)