R v Park, 99 CCC (3d) 1 (Lamer):

[20] Essentially, for there to be an “air of reality” to the defence of honest but mistaken belief in consent, the totality of the evidence for the accused must be reasonably and realistically capable of supporting that defence. Although there is not, strictly speaking, a requirement that the evidence be corroborated, that evidence must amount to something more than a bare assertion. There must be some support for it in the circumstances. The search for support in the whole body of evidence or circumstances can complement any insufficiency in legal terms of the accused’s testimony. The presence of “independent” evidence supporting the accused’s testimony will only have the effect of improving the chances of the defence. The judge’s role is limited to ascertaining whether the accused has discharged the evidentiary burden imposed by s. 265(4) of the Code.

[21] As a corollary, in the absence of some other evidence supporting such a defence, it is difficult to imagine situations in which there will be sufficient evidence to put the defence of mistaken belief to the jury where an accused does not testify (see R. v. Reddick, [1991] 1 S.C.R. 1086). Finally, for practical and policy reasons which shall be discussed below, courts must be wary of being too ready to put the mistaken belief defence to the jury. We must recall that it is the rare exception rather than the general rule that a sexual assault will have been committed by accident. As McLachlin J. observed in Osolin, supra, at p. 648, the defence of mistake of fact has only really received special attention in the context of sexual assault trials because of the frequency with which that defence is asserted without evidentiary foundation.
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[24] At the same time, it must always be remembered that consent, even if given at one point, may be withdrawn at any time. Consequently, it can be dangerous to assume that evidence capable of founding an honest belief on the part of the accused that the complainant would consent to sexual activity is informative of the real question at issue, which is whether the accused believed that the complainant in fact consented to that activity. An honest belief that the complainant would consent is not a defence to sexual assault where the accused is aware of, or wilfully blind or reckless as to, lack of consent at the time of the sexual activity. Only where an accused entertains an honest belief that the complainant actually does consent does this mistake render the sexual assault non-culpable. Absent some realistic showing of how earlier events could have influenced the accused’s honest perceptions of the complainant’s behaviour at the time of the actual assault, such evidence will not be capable of supporting, by itself, the defence of honest but mistaken belief in consent.

[25] Secondly, the fact that stories are diametrically opposed, as well as the degree to which they are opposed, is but one factor in the air of reality determination. Instances in which stories are diametrically opposed and where there is no air of reality are, in fact, specific applications of the air of reality threshold. They are not intrinsic to the test, itself. The question is not whether two stories are so diametrically opposed that there can be no air of reality to the defence of mistaken belief in consent. This approach ignores the presence of other factors, such as corroborative evidence, which may nonetheless justify putting the defence of honest mistake to the jury (see Bulmer, supra). Rather, we must bear in mind that neither the version of the facts given by the complainant nor that given by the accused is necessarily a full and complete account of what actually took place and, as such, a jury may decide not to believe certain parts of each person’s testimony. Thus, the question is whether, in the absence of other evidence lending an air of reality to the defence of honest mistake, a reasonable jury could cobble together some of the complainant’s evidence and some of the accused’s evidence to produce a sufficient basis for such a defence. Would the acceptance of one version necessarily involve the rejection of the other? Put another way, is it realistically possible for a properly instructed jury, acting judiciously, to splice some of each person’s evidence with respect to the encounter, and settle upon a reasonably coherent set of facts, supported by the evidence, that is capable of sustaining the defence of mistaken belief in consent? If the stories cannot realistically be spliced in such a manner, then the issue really is purely one of credibility — of consent or no consent — and the defence of mistaken belief in consent should not be put to the jury.

[26] To summarize, when the complainant and the accused give similar versions of the facts, and the only material contradiction is in their interpretation of what happened, then the defence of honest but mistaken belief in consent should generally be put to the jury, except in cases where the accused’s conduct demonstrates recklessness or wilful blindness to the absence of consent. On the other hand, courts have generally refused to put the defence of honest but mistaken belief in consent to the jury when the accused clearly bases his defence on voluntary consent, and he also testifies that the complainant was an active, eager or willing partner, whereas the complainant testifies that she vigorously resisted. In such cases, the question is generally simply one of credibility, of consent or no consent.
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30 Building upon this approach, it is possible to say that there is no air of reality to a particular defence, and therefore no need to put it to the jury, under the following circumstances:

(1) the totality of the evidence for the accused is incapable of amounting to the defence being sought; or
(2) the totality of the evidence for the accused is clearly, logically inconsistent with the totality of evidence which is not materially in dispute.

It goes without saying, of course, that these standards should be viewed realistically, and not evaluated according to purely speculative or hypothetical extremes. We must recall that a defence should not be put to the jury unless a reasonable jury, properly instructed and acting judiciously, could acquit on that basis.