Notes on the Bible, by Albert Barnes, [1834], at sacred-texts.com
It was the purpose of God that David should be anointed at this time as Saul's successor, and as the ancestor and the type of His Christ. It was not the purpose of God that Samuel should stir up a civil war, by setting up David as Saul's rival. Secrecy, therefore, was a necessary part of the transaction. But secrecy and concealment are not the same as duplicity and falsehood. Concealment of a good purpose, for a good purpose, is clearly justifiable. There is therefore nothing in the least inconsistent with truth in the occurrence here related. Compare Exo 7:16; Exo 8:1; Exo 9:13.
Trembled - There was evidently something unusual in Samuel's coming to Bethlehem; and the elders, knowing that Samuel was no longer at friendship with Saul, foreboded some evil.
Seven - i. e., including the three who had already passed. It appears that Jesse had eight sons; but in Ch1 2:13-15, only seven are ascribed to him.
We will not sit down ... - . literally, we will not turn round to sit at the table.
The Spirit ... came upon David - The exact phrase used of the Judges and Saul. See Sa1 10:6, note; Jdg 3:10, note; Jdg 6:34, note; Jdg 11:29, note; Jdg 14:18, note; Jdg 15:14, note.
The "evil" or "melancholy" spirit here spoken of was "the Spirit of God," or "of Jehovah," as being God's messenger and minister, sent by Him to execute His righteous purpose upon Saul (see Kg1 22:19-22 note).
The medicinal effects of music on the mind and body, especially as appeasing anger, and soothing and pacifying a troubled spirit, are well known. It is deeply interesting to have the youthful David thus brought before us, as using music for its highest purpose, that of turning the soul to the harmony of peace and love. We may infer that some of his Psalms, such e. g. as Psa 23:1-6, were already composed.
A mighty valiant man ... - David's reputation for courage, skill, discretion, and manly beauty, was already great. Since "the Spirit of the Lord came upon him," his natural qualities and powers had been greatly enhanced. His feat of killing the lion and the bear (see the marginal references) had been performed, like Samson's feats of strength Jdg 14:6, Jdg 14:19; Jdg 15:14, under the same supernatural influence, and was probably more or less known.
The difficulty of reconciling this verse with Sa1 17:55-58, is met thus: The words here are the ultimate sequence of David's first visit to Saul, and of his skill in music, and are therefore placed here; but they did not really come to pass until after David's victory over Goliath (see Sa1 18:2). It is quite conceivable that if David had only played once or twice to Saul, and then returned to his father's house for some months, Saul might not recognize him.