From wikiHow:
http://www.wikihow.com/Make-a-Citizen's-ArrestWarnings
* Every state in the US has different laws; guidelines are not the same, and your best option is often simply to observe and report. Crime Stoppers may pay you a reward.
* A citizen's arrest must be made during or immediately after the crime, or else it is illegal. In some states the crime must be a felony, only a Police Officer may make an arrest on a misdemeanor.
* By making a citizen's arrest, you're exposing yourself to possible lawsuits or criminal charges (e.g. impersonating police, false imprisonment, kidnapping, or wrongful arrest) if the wrong person is apprehended or if you violate a suspect's civil rights. This risk varies considerably from country to country, but in the U.S. in particular, a citizen's arrest is a legal minefield, and dealing with a suspect's lawyers is often more dangerous than apprehending the suspect.
* The State of North Carolina does not recognize a citizens arrest. (Andy Taylor and Gomer Pyle aside). In fact, you are more likely to be arrested for trying it than anyone you might bring in, due to the state's vigilante laws.
* Unless you want your last words to be 'I am making a citizens arrest' do not attempt to apprehend a suspect with a weapon, just get a good description to give to the police.
(This last point could very very big in this situation.)