Stood by him (epestē). Ingressive second aorist active indicative of ephistēmi, intransitive. This very form occurs in Luk 2:9 of the sudden appearance of the angel of the Lord to the shepherds. Page notes that this second aorist of ephistēmi occurs seven times in the Gospel of Luke, eight times in the Acts, and nowhere else in the N.T. Note also the same form apestē (departed from, from aphistēmi, stood off from) of the disappearance of the angel in Act 12:10.
In the cell (en tōi oikēmati). Literally, a dwelling place or habitation (from oikeō, to dwell, oikos, house), but here not the prison as a whole as in Thucydides, but the room in the prison (cell) where Peter was chained to the two guards. Old word, but only here in the N.T.
He smote Peter on the side (pataxas tēn pleuran tou Petrou). More exactly, “smote the side of Peter.” Strongly enough to wake Peter up who was sound asleep and yet not rouse the two guards. It was probably between 3 a.m. and 6 a.m., hours when changes in the guards were made.
Rise up (anasta). Short form (Koinéš) of anastēthi, second aorist active imperative of anistēmi, intransitive. So also Act 9:11 (Westcott and Hort text); Eph 5:14.
Fell off (exepesan). Second aorist active with a ending like first aorist of expiptō, old verb. This miracle was necessary if Peter was to escape without rousing the two guards.