A.T. Robertson Commentary Acts 9:24

"but their plot became known to Saul. And they watched the gates also day and night that they might kill him:" — Acts 9:24 (ASV)

Plot (επιβουλη). Old word for a plan (βουλη) against (επ) one. In the N.T. only in Acts (9:24; 20:3,19; 23:30).

They watched (παρετηρουντο). Imperfect middle indicative of παρατηρεω, common verb in late Greek for watching beside (παρα) or insidiously or on the sly as in Lu 6:7, they kept on watching by day and night to kill him. In 2 Corinthians 11:32 Paul says that the Ethnarch of Aretas "kept guard" (εφρουρε, imperfect active of φρουρεω) to seize him. Probably the Jews obtained the consent of the Ethnarch and had him appoint some of them as guards or watchers at the gate of the city.