Prosecuting a criminal trial is very different to running a civil law case as plaintiff. The prosecutor does not act as an adversary of the defendant in the same sense as a plaintiff’s barrister, but is required to present all the evidence available to him/her and to ensure that the prosecution is fair. The prosecutor is to be frank with the court about flaws in their case. Typically this is done by a person employed by a government body such as the Police or the Director of Public Prosecutions. Private prosecutions are also possible, circumventing all involvement of government bodies, although because of the standard of proof required, it would be extremely difficult to run a prosecution except with the investigative powers of the police available.
In dealing with crimes the police only lay charges if they believe that there is a reasonable chance of success at trial. In reaching that point they have to ensure not only that they can prove a crime has been committed and is causally linked to the defendant, but they must overcome a number of evidentiary hurdles.