It is a rare person who dares speak truth to power. Those who do are remembered, or ought to be:
Appianus, the Gymnasiarch of Alexandria, was sentenced to death because he had accused Emperor Commodus (180-192 AD), son of the great Marcus Aurelius, of misappropriating huge profits that belonged to Alexandrian ship owners and merchants to whom the supply of grain to Rome had been farmed out.
Before the execution, the Emperor Commodus summoned Appianus. This was their brief conversation:
Emperor Commodus: "Now, don't you know to whom you are speaking?"
Appianus: "I know, Appianus speaks to a tyrant."
Emperor: "No, to a king."
Appianus: "Don't say that, since it fitted your father, the divine Antoninus, to be an emperor. Listen, firstly, he was a philosopher, secondly, he was not greedy for money, and thirdly, he loved goodness. With you it's exactly the opposite: tyranny, hatred of goodness and lack of education."
The emperor ordered Appianus to be taken away (to be executed).
(The gymnasiarch was the supervisor of the gymnasium, a bath open to males who were born with or had acquired Greek citizenship. Commodus' father Marcus Aurelius [162-180 AD] had a good reputation in Egypt as a philosopher-emperor.)
This exchange between Emperor Commodus and Appianus is recounted in Jan Willem van Henten and Friedrich Avemarie, Martyrdom and Noble Death: Selected texts from Graeco-Roman, Jewish and Christian Antiquity, Routledge, 2002, pp. 38-40.
Another case of speaking truth to power occurred some years earlier between Emperor Vespasian (r. 69-79 AD) and his critic Helvidius Priscus, who had argued that the senate should decide the management of state finances. Their exchange was recounted by the Greek philosopher Epictetus (ca. 55-135 AD) in his Dissertations:
Priscus: You can banish me from the senate, but while I am a member of it, I must go into the house.
Vespasian: Well all right, go in but stay silent.
Priscus: Do not ask for my opinion and then I can keep quiet.
Vespasian: But I have to ask you.
Priscus: Then I must say what seems right to me.
Vespasian: But do that and I will execute you.
Priscus: I have never claimed to you that I was immortal. You do what is your part and I shall do mine. Yours is to kill me and mine to die unafraid. Yours is to banish me and mine to go into exile unsorrowing.
Helvidius Priscus was banished and later executed on Vespasian's order.
Quoted by Paula James, "The language of dissent," in Janet Huskinson, Experiencing Rome: Culture, Identity and Power in the Roman Empire, Routledge, 1999, p. 293.