The repentance of Ahab resembles that of the Ninevites Jonah 3:5. It has the same outward signs - fasting and sackcloth - and it has much the same inward character. It springs, not from love, nor from hatred of sin, but from fear of the consequences of sin. It is thus, although sincere and real while it lasts, shallow and exceedingly short-lived. God, however, to mark His readiness to receive the sinner who turns to Him, accepted the imperfect offering (as He likewise accepted the penitence of the Ninevites), and allowed it to delay the execution of the sentence 1 Kings 21:29. So the penitence of the Ninevites put off the fall of Nineveh for a century.
And lay in sackcloth - In this particular he seems to have gone beyond the usual practice. We do not read elsewhere of mourners passing the night in sackcloth.
And went softly - “As if he had no heart to go about any business” (Patrick).