A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
gen 20:1ABRAHAM'S DENIAL OF HIS WIFE. (Gen. 20:1-18)
Abraham journeyed from thence . . . and dwelled between Kadesh and Shur--Leaving the encampment, he migrated to the southern border of Canaan. In the neighborhood of Gerar was a very rich and well-watered pasture land.
gen 20:2Abraham said of Sarah his wife, She is my sister--Fear of the people among whom he was, tempted him to equivocate. His conduct was highly culpable. It was deceit, deliberate and premeditated--there was no sudden pressure upon him--it was the second offense of the kind [see on Gen 12:13] --it was a distrust of God every way surprising, and it was calculated to produce injurious effects on the heathen around. Its mischievous tendency was not long in being developed.
Abimelech (father-king) . . . sent and took Sarah--to be one of his wives, in the exercise of a privilege claimed by Eastern sovereigns, already explained (see on Gen 12:15).
gen 20:3But God came to Abimelech in a dream--In early times a dream was often made the medium of communicating important truths; and this method was adopted for the preservation of Sarah.
gen 20:9Then Abimelech called Abraham, and said . . . What hast thou done?--In what a humiliating plight does the patriarch now appear--he, a servant of the true God, rebuked by a heathen prince. Who would not rather be in the place of Abimelech than of the honored but sadly offending patriarch! What a dignified attitude is that of the king--calmly and justly reproving the sin of the patriarch, but respecting his person and heaping coals of fire on his head by the liberal presents made to him.
gen 20:11And Abraham said . . . I thought, Surely the fear of God is not in this place--From the horrible vices of Sodom he seems to have taken up the impression that all other cities of Canaan were equally corrupt. There might have been few or none who feared God, but what a sad thing when men of the world show a higher sense of honor and a greater abhorrence of crimes than a true worshipper!
gen 20:12yet indeed she is my sister--(See on Gen 11:31). What a poor defense Abraham made. The statement absolved him from the charge of direct and absolute falsehood, but he had told a moral untruth because there was an intention to deceive (compare Gen 12:11-13). "Honesty is always the best policy." Abraham's life would have been as well protected without the fraud as with it: and what shame to himself, what distrust to God, what dishonor to religion might have been prevented! "Let us speak truth every man to his neighbor" [Zac 8:16; Eph 4:25].