Adam Clarke on Luke 20:20

They watched him - Παρατηρησαντες, Insidiously watching. See on Luk 14:1 (note).

Spies - Εγκαθετους, from εν, in, and καθιημι, I let down, to set in ambush. One who crouches in some secret place to spy, listen, catch, or hurt. Hesychius explains the word by ενεδρευοντες, those who lie in wait, or in ambush, to surprise and slay. Josephus uses the word to signify a person bribed for a particular purpose. See War, b. ii. c. ii. s. 5, and b. vi. c. v. s. 2. No doubt the persons mentioned in the text were men of the basest principles, and were hired by the malicious Pharisees to do what they attempted in vain to perform.