The Sicariis most notable tactic was the use of short daggers to kill people. Although they were not terrorists in the modern sense, this method of murdering people in crowded places before slipping away did cause extreme anxiety among surrounding onlookers, and thus terrorize them.
As political scientist and terrorist expert David C. Rapaport has pointed out, the Sicarii were distinct in primarily targeting other Jews considered to be either collaborators or quiescent in the face of Roman rule.
They attacked, in particular, Jewish notables and elites associated with the priesthood. This strategy distinguishes them from the Zealots, who aimed their violence against Romans.
These tactics were described by Josephus as beginning in the CE 50s:
… a different type of bandits sprang up in Jersualem, the so-called sicarii, who murdered men in broad daylight in the heart of the city. Especially during the festivals they would mingle with the crowd, carrying short daggers concealed under their clothing, with which they stabbed their enemies. Then when they fell, the murderers would join in the cries of indignation and, through this plausible behavior, avoided discovery. (Quoted in Richard A. Horsley, "The Sicarii: Ancient Jewish "Terrorists," The Journal of Religion, October 1979.)
The Sicarii operated primarily in the urban environment of Jerusalem, including within the Temple. However, they also committed attacks in villages, which they also raided for plunder and set on fire in order to create fear among Jews who acquiesced or collaborated with Roman rule. They also kidnapped notables or others as leverage for the release of their own members held prisoner.