Defence counsel must be vigilant to guard against improper Crown opening addresses. At a minimum, an improper Crown opening will warrant a corrective instruction from the judge. In more egregious cases, a Defence application for a mistrial may be appropriate.
When listening to the Crown’s opening address, Defence counsel may want to keep in mind the following comments from a unanimous Court of Appeal in R. v. Mallory,  O.J. No. 236 at para. 338:
It is well established that the opening address is not the appropriate forum for argument, invective, or opinion. The Crown should use the opening address to introduce the parties, explain the process, and provide a general overview of the evidence that the Crown anticipates calling in support of its case… Simply put, "the Crown's opening address should be impartial and fair, a brief outline of the evidence that the Crown intends to call": R. v. Griffin at para. 31. At the opening of the trial the rules constraining the Crown "should apply with even more vigour" than at the closing when by then the jurors have heard and seen all about the case: R. v. Griffin at para. 23. (citations omitted)