From: The Golden Sayings of Epictetus (translated by Hastings Crossley) - from Project Gutenberg.
Remember that thou art an actor in a play, and of such sort as the Author chooses, whether long or short. If it be his good pleasure to assign thee the part of a beggar, a ruler, or a simple citizen, thine it is to play it fitly. For thy business is to act the part assigned thee, well: to choose it, is another's.
Lose no time in setting before you a certain stamp of character and behaviour both when by yourself and in company with others. Let silence be your general rule; or say only what is necessary and in few words. We shall, however, when occasion demands, enter into discourse sparingly. avoiding common topics as gladiators, horse-races, athletes; and the perpetual talk about food and drink. Above all avoid speaking of persons, either in way of praise or blame, or comparison.
If you can, win over the conversation of your company to what it should be by your own. But if you find yourself cut off without escape among strangers and aliens, be silent.