Epictetus, Golden Sayings 164

Lose no time in setting before you a certain stamp of character and behavior, 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.