Legal Authority, Limitations, and Relevant New Jersey Law
Where authority comes from
You have no police powers. Your authority is the client's property rights plus rights held by any private citizen.
Arrest and detention. New Jersey's citizen-arrest rule is narrow: a private person may arrest without a warrant for a breach of the peace committed in your presence, or for a felony actually committed when you have reasonable grounds to believe the person did it. A detention must rest on probable cause of a crime, be brief, use the least force necessary, and end with police notification. Detaining to "teach a lesson" or to take a statement is false imprisonment.
Use of force. Under N.J.S.A. 2C:3-4 through 2C:3-6, force must be proportional. Deadly force is limited to preventing death or serious bodily harm; it is never available merely to protect property. Force stops when resistance stops.
Search. There is no general right to search. Lawful bases: voluntary consent (which may be withdrawn), plain view, abandoned property, and search incident to a lawful arrest limited to the person and reachable area. Post orders may restrict you further but never expand what law allows.
Statements. Miranda warnings bind government agents, not private security working for a client - a frequent trap. Statements must still be voluntary; coercion is illegal and worthless.
Trespass. You may ask a person to leave private property; refusal may be defiant trespass, N.J.S.A. 2C:18-3. Call police rather than fight.
Firearms. Armed assignments require the state permits and training the State Police require; carry only when authorized and certified.
Rule of thumb
Ask: "Could an ordinary citizen with no badge lawfully do this?" If not, do not.