Jamieson, Fausset and Brown on Deuteronomy 13:9

9. thou shalt surely kill him--not hastily, or in a private manner, but after trial and conviction; and his relative, as informer, was to cast the first stone (see on De 17:2; Ac 7:58). It is manifest that what was done in secret could not be legally proved by a single informer; and hence Jewish writers say that spies were set in some private part of the house, to hear the conversation and watch the conduct of a person suspected of idolatrous tendencies.