Epictetus was born a slave in Hierapolis (modern-day Pamukkale, Turkey) and made his way to Rome as the property of Epaphroditus, secretary to Nero. Granted freedom in his late twenties or early thirties, he studied under the Stoic Musonius Rufus, opened a school in Rome, was expelled along with the other philosophers by Domitian in 93 CE, and re-established himself in Nicopolis on the Adriatic coast. He never wrote anything down. The works we have are his student Arrian's notes from the lectures — a fact that gives the Discourses their unmistakable voice: live, interrogative, often impatient, frequently brutal.
His characteristic move is the elenchus inherited from Socrates: get the student to commit to a position, then surface the behaviour that contradicts it. Show me a Stoic, he says in Discourses 2.19 — show me someone who is sick and happy, who is in disgrace and happy. You cannot? Then show me one in the making, one who has set his feet on the path. The standard is the sage, but the daily target is the prokopton — the one making progress.
Three doctrinal moves are particular to Epictetus. First, the discipline of assent: catching impressions before granting them belief, which is the Stoic mechanism Albert Ellis (REBT, 1950s) and then Aaron Beck (cognitive therapy, 1960s) rediscovered as the foundation of cognitive-behavioural therapy — Ellis explicitly named Epictetus as a precursor. Second, the dichotomy of control, articulated more sharply than in any earlier Stoic — what is up to us, and what is not. Third, prohairesis, the faculty of choice, which Epictetus treats as the seat of the self. Everything else can be taken from you; this cannot.
He is the Stoic who most directly addresses the gap between knowing the doctrine and living it. He has no patience for the student who can recite Chrysippus but cannot keep his temper. His standard line, which Donald Robertson and others have made central to modern Stoicism, is: do not say you are studying Stoicism. Say what you are doing differently. Show me your shoulders.