R. v. A.M., 2014 ONCA 769, the Ontario Court of Appeal helpfully summarized these principles.
1. Every witness, irrespective of age, is an individual whose credibility and evidence should be assessed according to criteria appropriate to his or her mental development, understanding, and ability to communicate.
2. Where an adult testifies about events that occurred when she was a child, her credibility should be assessed according to the criteria applicable to adult witnesses. However, the presence of inconsistencies, especially on peripheral matters such as time and location, should be considered in the context of her age at the time the events about which she is testifying occurred.
3. One of the most valuable means of assessing witness credibility is to examine the consistency between what the witness said in the witness box and what she has said on other occasions, whether or not under oath. Inconsistencies may emerge in a witness’s testimony at trial, or between their trial testimony and statements previously given. Inconsistencies may also emerge from things said differently at different times, or from omitting to refer to certain events at one time while referring to them on other occasions.
4. Inconsistencies vary in their nature and importance. Some are minor, others are not. Some concern material issues, others peripheral subjects. Where an inconsistency involves something material about which an honest witness is unlikely to be mistaken, the inconsistency may demonstrate a carelessness with the truth about which the trier of fact should be concerned.
5. A trial judge giving reasons for judgment is neither under the obligation to review and resolve every inconsistency in a witness’s evidence, nor respond to every argument advanced by counsel. That said, a trial judge should address and explain how she or he has resolved major inconsistencies in the evidence of material witnesses.
6. Prior consistent statements of a witness are not admissible for their truth. Mere repetition of a story on a prior occasion does not make the in-court description of the events any more credible or reliable.
7. In reviewing reasons for sufficiency, we must ask ourselves whether the reasons, taken as a whole and considered with the evidentiary record, the submissions of counsel and the live issues at trial reveal the basis for the verdict reached.
8. Where a case turns largely on determinations of credibility, the sufficiency of reasons must be considered in light of the deference generally afforded to trial judges on credibility findings. It is rare for deficiencies in a trial judge’s credibility analysis, as expressed in the reasons for judgment, to warrant appellate intervention.
9. Nevertheless, the failure of a trial judge to sufficiently articulate how credibility and reliability concerns are resolved may constitute reversible error. After all, an accused is entitled to know why the trial judge had no reasonable doubt about his or her guilt.
10. Similarly, we take it as self-evident that a legal error made in the assessment of credibility may displace the deference usually afforded to a trial judge’s credibility assessment and may require appellate intervention.
11. Finally, when evaluating a trial judge’s credibility analysis, there is no principled reason to distinguish between cases involving oath against oath from those in which no competing oath has been offered.
Note: This is useful information for a lawyer when defending Criminal Charges such as Sexual Assaults in Court.