Opened his mouth (anoixas to stoma). Solemn formula for beginning his address (Act 8:35; Act 18:14; Mat 5:2; Mat 13:35). But also good elocution for the speaker.
I perceive (katalambanomai). Aoristic present middle of katalambanō, to take hold of, the middle noting mental action, to lay hold with the mind (Act 4:13; Act 10:34; Act 25:25; Eph 3:18). It had been a difficult thing for Peter to grasp, but now “of a truth” (ep' alētheias) the light has cleared away the fogs. It was not until Peter had crossed the threshold of the house of Cornelius in the new environment and standpoint that he sees this new and great truth.
Respecter of persons (prosōpolēmptēs). This compound occurs only here and in Chrysostom. It is composed of prosōpon face or person (pros and ops, before the eye or face) and lambanō. The abstract form prosōpolēmpsia occurs in Jam 2:1 (also Rom 2:11; Eph 6:9; Col 3:25) and the verb prosōpolempteō in Jam 2:9. The separate phrase (lambanein prosōpon) occurs in Luk 20:21; Gal 2:6. The phrase was already in the lxx (Deuteronomy 10:17; 2 Chronicles 19:7; Ps 82:6). Luke has simply combined the two words into one compound one. The idea is to pay regard to one’s looks or circumstances rather than to his intrinsic character. The Jews had come to feel that they were the favourites of God and actually sons of the kingdom of heaven because they were descendants of Abraham. John the Baptist rebuked them for this fallacy.