The Crown must never been seen to improperly influence the proceedings or taint the evidence. Not only is that contrary to their ethical obligations and role, it will undoubtedly negatively affect their case. Crown interference, even through so-called preparation, can result in a Crown Attorney becoming a witness to the own proceeding or worse still a stay of proceeding for an abuse of process.
Imagine the following scenario: A Crown Attorney meets with a witness alone prior to trial to prepare them for their testimony and anticipated cross-examination on details that are unhelpful to their case. After such a meeting the witness amends their anticipated evidence to details that are more favourable to the Crown. As an example, imagine in this scenario the description of an alleged bank robber went from “6’2, tanned, and speaking with a Canadian accent” pre-meeting to “5’10-6′, caucasian, tattoo of a spider on his neck, and with a strong British accent.” Now imagine that the latter description is far more descriptive of the accused, whereas the former is exclusionary.
Any defence lawyer in the above situation would be required to ask “What happened in that private meeting that resulted in such a drastic change? What details were presented to the witness? What was said? Why was the interview not video recorded? How did this Crown Attorney seemingly influence their evidence to one far more favourable to the case?
Crowns are not immune from suggestions of impropriety or influence from the defence. Nor are they immune from becoming a witness themselves if circumstances arise that make their participation material to a change in evidence. Nor are they immune from their participation resulting in a stay of proceeding for irreparably tainting the case against the accused. This is why Crowns are taught that meeting witnesses in private is to be avoided at all costs and to use the resources at their disposal (such as police interviews) in obtaining anticipated evidence in a manner that is untainted from their own involvement.
Insofar as empowering victims with familiarity of the process, there are specific governmental bodies dedicated to this who will meet with victims in advance. In Ontario, Victim Witness Services provides extensive support to explain the process and how to behave in Court. Notwithstanding, Crown Attorneys will still out of respect and courtesy to the alleged victims will meet in advance to introduce themselves and ask basic questions that would not seem to be influential to their evidence.