If I laughed on them they believed it not - There is considerable variety in the interpretation of this member of the verse. Dr. Good renders it, "I smiled upon them, and they were gay." Herder, If I laughed at them, they were not offended." Coverdale," When I laughed, they knew well it was not earnest." Schultens, "I will laugh at them, they are not secure." But Rosenmuller, Jun. et Trem., Noyes and Umbreit, accord with the sense given in our common translation. The Hebrew literally is, "Should I laugh upon them, they did not confide;" and, according to Rosenmuller, the meaning is, "Such was the reverence for my gravity, that if at any time I relaxed in my severity of manner, they would scarcely believe it, nor did they omit any of their reverence toward me, as if familiarity with the great should produce contempt." Grotius explains it to mean, "Even my jests, they thought, contained something serious." The word used here, however (שׂחק śâchaq), means not only to laugh or smile upon, but; to laugh at, or deride; Psalm 52:6; Job 30:1; compare Job 5:22; Job 39:7; Job 22:19. It seems to me, that the sense is that so great was his influence, that he was able to control them even with a smile, without saying a word; that if, when a measure was proposed in debate, he should even smile, though he said nothing, they would have no confidence in it, but would at once abandon it as unwise. No higher influence than this can be well conceived, and this exposition accords with the general course of remark, where Job traces along the various degrees of his influence until he comes to this, the highest of them all.
And the light of my countenance they cast not down - His smile of favor on an undertaking, or his smile at the weakness or lack of wisdom of any thing proposed, they could not resist. It settled the matter. They had not power by their arguments or moral courage to resist him even if he did not say a word, or even to change the aspect of his countenance. A look, a token of approbation or disapprobation from him, was enough.

I laughed on them, they believed it not - Similar to that expression in the Gospel, Luke 24:41 : And while they believed not for joy, and wondered, he said - . Our version is sufficiently perspicuous, and gives the true sense of the original, only it should be read in the indicative and not in the subjunctive mood: I laughed on them - they believed it not. We have a similar phrase: The news was too good to be true.
The light of my countenance - This evidence of my benevolence and regard. A smile is, metaphorically, the light of the countenance.
They cast not down - They gave me no occasion to change my sentiments or feelings towards them. I could still smile upon them, and they were then worthy of my approbation. Their change he refers to in the beginning of the next chapter.

[If] I (q) laughed on them, they believed [it] not; and the light of my countenance they (r) cast not down.
(q) That is, they thought it not to be a rest, or they did not think that I would condescend to them.
(r) They were afraid to offend me and cause me to be angry.

If I laughed on them, they believed it not,.... Not that he at any time laughed at them, by way of derision; but when in a cheerful frame of mind, or in a merry mood, he used freedom and familiarity, and jested with them; but they could not believe that he did jest, or was in jest, he being a man always of such gravity and seriousness, that they concluded the smile on his countenance, and the pleasant turn of his expression, had a serious meaning in them; or such familiarity with them was so pleasing to them, that they could scarcely for joy believe that he did condescend to indulge such an air of pleasantry: or as Mr. Broughton renders it, and so some others to the same sense, "they would not be bold" (i); familiarity with them did not breed contempt, as it sometimes does; they did not presume upon it, and grow bold and insolent, and make him their equal, and jest with him again; but still there was an awe upon them, and they behaved with reverence to him; and to show how great it was is the design of the expression:
and the light of my countenance they cast not down; they did not ruffle his mind, or disturb the serenity of it; or cause him to change his countenance, through any bold and indecent behaviour towards him, encouraged by the freedom and pleasantry he used with them; they did not put him to shame, or provoke him to anger and displeasure by any unbecoming deportment; they kept their distance, they did not detract from his authority and majesty, or in the least lessen that, but behaved with the same reverence and regard to him they ever did; see Genesis 4:6.
(i) "non tamen sibi sumebant audaciam", Michaelis; "neque tam audaces fiunt", Reimar apud Schultens.

When I relaxed from my wonted gravity (a virtue much esteemed in the East) and smiled, they could hardly credit it; and yet, notwithstanding my condescension, they did not cast aside reverence for my gravity. But the parallelism is better in UMBREIT'S translation, "I smiled kindly on those who trusted not," that is, in times of danger I cheered those in despondency. And they could not cast down (by their despondency) my serenity of countenance (flowing from trust in God) (Proverbs 16:15; Psalm 104:15). The opposite phrase (Genesis 4:5-6). "Gravity" cannot well be meant by "light of countenance."

Laughed - Carried myself so familiarly with them, that they could scarce believe their eyes and ears. Cast not down - They were cautious not to give me any occasion to change my countenance towards them.