Notes on the Bible, by Albert Barnes, [1834], at sacred-texts.com
A fountain - Probably, the fine spring Ain-Jalud. It is impossible to say what the special circumstances were which led to the struggle between Israel and the Philistines taking place so far north as the plain of Jezreel. Possibly it was connected with some movements of the Aramaic tribes to the north of Palestine. See 2 Sam. 8.
The lords - See Jdg 3:3 note, as distinguished from ordinary "princes" Sa1 29:3. The military divisions of the Philistine army were by hundreds and by thousands, like those of the Israelites Sa1 8:12. David and his men formed a body-guard to Achish Sa1 28:2.
He fell unto me - The regular word for deserting and going over to the other side. See Jer 37:13; Jer 38:19.
As the Lord liveth - The swearing by Yahweh seems strange in the mouth of a Philistine. But probably not the very words, but only the sense of this and such like speeches, is preserved.
See Sa1 29:10 note.
With thy master's servants - The clue to this may be found in Ch1 12:19-21, where it appears that a considerable number of Manassites "fell" to David just at this time, and went back with him to Ziklag. It is therefore to these new comers that Achish applies the expression. It is impossible not to recognize here a merciful interposition of Providence, by which David was not only saved from fighting against his king and country, but sent home just in time to recover his wives and property from the Amalekites 1 Sam. 30. That David maintained his position by subtlety and falsehood, which were the invariable characteristics of his age and nation, is not in the least to be wondered at. No sanction is given by this narrative to the use of falsehood.