Enchiridion · §17

μέμνησο ὅτι ὑποκριτὴς εἶ δράματος, οἵου ἂν θέλῃ ὁ διδάσκαλος· ἂν βραχύ, βραχέος· ἂν μακρόν, μακροῦ· ἂν πτωχὸν ὑποκρίνεσθαί σε θέλῃ, ἵνα καὶ τοῦτον εὐφυῶς ὑποκρίνῃ· ἂν χωλόν, ἂν ἄρχοντα, ἂν ἰδιώτην. σὸν γὰρ τοῦτο ἔργον, τὸ δοθὲν πρόσωπον καλῶς ὑποκρίνεσθαι· ἐκλέξασθαι δὲ αὐτὸ ἄλλου.

memnēso hoti hupokritēs ei dramatos, hoiou an thelē ho didaskalos; an brachu, bracheos; an makron, makrou; an ptōchon hupokrinesthai se thelē, hina kai touton euphyōs hupokrinē; an chōlon, an archonta, an idiōtēn. son gar touto ergon, to dothen prosōpon kalōs hupokrinesthai.

Remember that you are an actor in a play, the character of which is determined by the Author: if short, a short one; if long, a long one. If the Author wants you to play a poor man, play even this role skillfully; if a cripple, if an official, if an ordinary citizen. For your task is to play the assigned role well. But the choosing of the role belongs to another.

— Enchiridion 17

Source: Greek (Schenkl/Oldfather critical edition tradition) + Long 1877 English translation