Most criminal cases are started when someone complains to the police and, following investigation, the police lay charges.
The police will lay a charge when they conclude there are grounds to believe an offence has occurred, they believe they know who is responsible and there are no good reasons not to proceed. Sometimes the police decline to lay a charge even though it is possible a valid charge could be laid. This is not a bad thing – the police have a discretion and so long as the discretion is exercised impartially and fairly no problem arise.
That said, anyone can go before a Justice of the Peace and seek to have a charge laid against someone. That means even if the police refuse to proceed an individual can. Most commonly these are charges for assault although almost any criminal case can start this way.
These type of prosecutions usually come up in the context of a toxic family law dispute. So, for example, charges are laid against a man for assaulting a woman – and the man then decides it would make sense to charge the woman because she hit him during a fight sometime in the past. While such a step may well be legal it is almost never prudent. Bringing more criminal issues into a dispute usually just makes things worse. A better approach is to reference the earlier assault by the woman as being a good reason for the Crown to resolve the current assault charges against the man.
That said, if someone wants to try to have a charge laid through a private prosecution they are quite entitled so to do.
There is a two step process to laying a private information. First, the complaint appears before a Justice of the Peace and gives a sworn statement. That statement must set out the elements of a recognized criminal charge. The Justice of the Peace, once satisfied there is a charge available in proper form accepts the charge and adjourns the matter to a second stage hearing.
The second stage hearing, usually heard before a Justice of the Peace (although sometimes a judge conducts the hearing) is far more detailed and takes place in private (the accused is not notified) but the Crown is there and asks questions. The Crown often probes the case quite deeply and sometimes someone seeking to lay a charge finds that offensive – they should not. The Crown is trying to ensure that only legitimate cases go ahead and a careful analysis of the case is totally appropriate. At this second stage hearing there is a determination if there really is a basis for the prosecution and whether it's in the public interest to proceed.
The Crown can step in and stay any criminal charge at any time. Even if the Justice of the Peace finds there is a basis for the charge to go ahead the Crown can stay the proceedings.
That said, if there is good evidence of, say, an assault the charge will formally go ahead. In that case the accused will be served with the charge by the police and the case will proceed in criminal court.