Another decision from a Canadian court in Ontario pertaining to the issue of collateral evidence.
R. v. Emmanuel, 1997, ON General Division:
8 I shall begin by saying that, in my view, and I don't think there is much doubt about it, credibility is always a collateral issue. The collateral evidence rule, put simply, is that the cross-examiner can ask just about any question he wishes relating to credibility but is stuck with the response. In this case Mr. Hinkson does not wish to be stuck with the response.
9 An exception to the collateral evidence rule is, of course, that provided by the Canada Evidence Act which permits a witness to be cross-examined as to a previous conviction and, in the event of denial, enables the questioner to prove that conviction in the normal way. Previous conviction is as much a collateral issue as previous conduct which did not result in a conviction, but there is a special statutory exception.
10 The collateral evidence rule is judge-made law and, in my view, if strictly applied, may prevent an accused person from making full answer and defence and, therefore, one ought not dismiss offhand a line of questioning on the basis that it is simply collateral. The collateral evidence rule is simply a rule of practical application to prevent cases from wandering on forever and ever.
Hopefully some of these cases provide some additional insight and clarity on the topic.