A witness’s interest in the outcome of a proceeding is a relevant consideration when assessing credibility (R. v. Laboucan,  1 S.C.R. 397 at para. 11). It is impermissible, however, to assume that a witness is going to lie under oath as a result of his or her interest. As Justice Charon states, speaking for a unanimous Supreme Court of Canada in Laboucan at para. 11:
…A trier of fact, however, should not place undue weight on the status of a person in the proceedings as a factor going to credibility. For example, it would be improper to base a finding of credibility regarding a parent's or a spouse's testimony solely on the basis of the witness's relationship to the complainant or to the accused. Regard should be given to all relevant factors in assessing credibility.
Of particular concern to the fairness of a trial is the pernicious and impermissible assumption that an accused will, as a matter of course, lie to secure an acquittal. Justice Charon acknowledges in Laboucan that the “common sense proposition that a witness's interest in the proceedings may have an impact on credibility also applies to an accused person who testifies in his or her defence” (para. 12). However, she cautions that “in most cases” it is ‘simply unhelpful’ to consider an accused’s interest in the outcome of the proceeding when assessing his or her credibility as a witness (para. 14). Justice Charon states that “as a general rule, triers of fact would be well advised to avoid that path altogether, lest they unwittingly err by making the impermissible assumption that the accused will lie to secure an acquittal” (para. 14).
As Justice Charon explains at para. 12:
… both innocent and guilty accused have an interest in not being convicted. …Therefore, any assumption that an accused will lie to secure his or her acquittal flies in the face of the presumption of innocence, as an innocent person, presumably, need only tell the truth to achieve this outcome. In R. v. B. (L.) (1993), 13 O.R. (3d) 796 (C.A.), Arbour J.A. (as she then was) succinctly described the inherent danger in considering the accused's motive arising from his or her interest in the outcome of the trial. In an often-quoted passage, she stated as follows (at pp. 798-99):
It falls into the impermissible assumption that the accused will lie to secure his acquittal, simply because, as an accused, his interest in the outcome dictates that course of action. This flies in the face of the presumption of innocence and creates an almost insurmountable disadvantage for the accused. The accused is obviously interested in being acquitted. In order to achieve that result he may have to testify to answer the case put forward by the prosecution. However, it cannot be assumed that the accused must lie in order to be acquitted, unless his guilt is no longer an open question. If the trial judge comes to the conclusion that the accused did not tell the truth in his evidence, the accused's interest in securing his acquittal may be the most plausible explanation for the lie. The explanation for a lie, however, cannot be turned into an assumption that one will occur. [Emphasis added.]