A good starting point for anyone seeking to gain a better appreciation of the role of the Crown Attorney in the Canadian criminal justice system is the following frequently cited passage from Rand, J. in R. v. Boucher,  S.C.R. 16:
It cannot be over-emphasized that the purpose of a criminal prosecution is not to obtain a conviction, it is to lay before a jury what the Crown considers to be credible evidence relevant to what is alleged to be a crime. Counsel have a duty to see that all available legal proof of the facts is presented: it should be done firmly and pressed to its legitimate strength but it must also be done fairly. The role of prosecutor excludes any notion of winning or losing; his function is a matter of public duty than which in civil life there can be none charged with greater personal responsibility. It is to be efficiently performed with an ingrained sense of the dignity, the seriousness and the justness of judicial proceedings.
The principles set out in Boucher guide the Crown's decision-making at each stage of the criminal justice process.
The underlying theme is that of fairness with respect to the conduct of a criminal prosecution.
During a trial, this passage may be useful for defence counsel when seeking judicial intervention to address improper Crown conduct that may jeopardize a client’s right to a fair trial (for example, an improper Crown opening/closing to a jury or an improper Crown cross-examination of a client).