False imprisonment is a crime in which you are accused of forcibly holding someone against his or her will. Since there are situations in which it may be legal to detain someone, the court will not just take your accuser's word for it. There are elements of the charge that the prosecution will have to prove. For example, the prosecution will have to prove that you detained the defendant:
The first thing your accuser will need to prove is that you intended to imprison him or her. This prevents the law from punishing those who unintentionally imprison others. For example, it would be unfair to convict a storeowner of false imprisonment if he or she locks a person in the store for the night if she did so believing that that everybody was out of the store.
As with most legal issues, there is an exception to this rule. You may be charged and convicted of false imprisonment if you intended to lockup an individual but ended up locking somebody else. For example, if you wanted to lockup your troublesome teenager but ended up locking up his or her friend.
Against His or Her Will
You are deemed to have detained the plaintiff against his or her will if your actions or words made him or her believe that he or she was being detained. This includes physical confinement, for example by closing all exists or tying the person, as well as using force (such as beatings) or threats to keep the person confined.
The threat doesn't have to be real; it is enough that any reasonable person would have believed it. Consider a situation in which you threaten a person not to get out of your office while holding up a toy gun. In such a case, a reasonable person would believe in your threats, so it is considered false imprisonment.
Lastly, the courts will evaluate whether or not the detention had a legal justification. Apart from detention by law enforcement authorities, there are situations in which it is lawful for you (as a civilian) to detain another person. For example, if you are a shopkeeper, then you can use the Shopkeeper's Privilege to detain a shoplifter until the police arrive. In most jurisdictions, it will be up to the judge or jury to decide whether you had lawful cause to detain the plaintiff.
If you believe that the detention was necessary and lawful, then some of the defenses your legal team is likely to adopt include citizens arrest, detention of a minor (under parental knowledge) or shoplifters privilege. An experienced criminal defense attorney (such as Eric Schurman, Attorney at Law) will advise you on the best defense to use for your situation.