Quotations about   behavior

Precisely in trifles, wherein a man is off his guard, does he show his character, and then we are often able at our leisure to observe in small actions or mere mannerisms the boundless egoism which has not the slightest regard for others and in matters of importance does not afterwards deny itself, although it is disguised. We should never miss such an opportunity. If in the petty affairs and circumstances of everyday life, in the things to which the de minimis lex non curat applies, a man acts inconsiderately, seeking merely his own advantage or convenience to the disadvantage of others; if he appropriates that which exists for everybody; then we may be sure that there is no justice in his heart, but that he would be a scoundrel even on a large scale if his hands were not tied by law and authority; we should not trust him across our threshold. Indeed, whoever boldly breaks the laws of his own circle will also break those of the State whenever he can do so without risk.

Arthur Schopenhauer (1788-1860) German philosopher
Parerga and Paralipomena, vol. 1, “Aphorisms on the Wisdom of Life,” part 2 “Counsels and Maxims,” ch. C “Our Attitude toward Others,” sec. 29 (1851) [tr. Payne (1974)]
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The Latin means, "The law is not concerned with trifles." Alternate translations:

A man shows his character just in the way in which he deals with trifles, -- for then he is off his guard. This will often afford a good opportunity of observing the boundless egoism of man's nature, and his total lack of consideration for others; and if these defects show themselves in small things, or merely in his general demeanor, you will find that they also underlie his action in matters of importance, although he may disguise the fact. This is an opportunity which should not be missed. If in the little affairs of every day, -- the trifles of life, those matters to which the rule de minimis non applies, -- a man is inconsiderate and seeks only what is advantageous or convenient to himself, to the prejudice of others' rights; if he appropriates to himself that which belongs to all alike, you may be sure there is no justice in his heart, and that he would be a scoundrel on a wholesale scale, only that law and compulsion bind his hands. Do not trust him beyond your door. He who is not afraid to break the laws of his own private circle, will break those of the State when he can do so with impunity.
[tr. Saunders (1890), published separately as Schopenhauer, Counsels and Maxims, ch. 3 "Our Relation to Others," sec. 29]

Men best show their character in trifles, where they are not on their guard. It is in insignificant matters, and in the simplest habits, that we often see the boundless egotism which pays no regard to the feeling of others, and denies nothing to itself.
[In Tryon Edwards, A Dictionary of Thoughts, "Character" (1891); this is the version quoted most often.]