Vincent's NT Word Studies
24. Laying await (epiboulh). So rendered by A.V. wherever it occurs, viz., ch. xx. 3, 19; xxiii. 30; but properly changed by Rev., in every case, to plot. "Laying await" refers rather to the execution of the plot than to the plot itself.Watched. See on Mark iii. 2. Imperfect: they were or kept watching, day and night.
Robertson's NT Word Studies
9:24 {Plot} (epiboule). Old word for a plan (boule) against (epi) one. In the N.T. only in Acts (#9:24; 20:3,19; 23:30). {They watched} (pareterounto). Imperfect middle indicative of paraterew, common verb in late Greek for watching beside (para) or insidiously or on the sly as in #Lu 6:7, they kept on watching by day and night to kill him. In #2Co 11:32 Paul says that the Ethnarch of Aretas "kept guard" (efrourei, imperfect active of frourew) 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.