Appearance

Appearance matters because it and manners leave a lasting impression. Neither of them should ever be bad.
“Men in general judge more by the sense of sight than by the sense of touch, because everyone can see but few can test by feeling. Everyone sees what you seem to be, few know what you really are; and those few do not dare take a stand against the general opinion.” 

― Niccolò Machiavelli, The Prince

Never Interrupt Anyone

Never interrupt anyone who is speaking; it is quite rude to officiously supply a name or date about which another hesitates, unless you are asked to do so. Another gross breach of etiquette is to anticipate the point of a story which another person is reciting, or to take it from his lips to finish it in your own language. Some persons plead as an excuse for this breach of etiquette, that the reciter was spoiling a good story by a bad manner, but this does not mend the matter. It is surely rude to give a man to understand that you do not consider him capable of finishing an anecdote that he has commenced.
A Gentleman’s Guide to Etiquette by Cecil B. Hartley