What is the proper approach to assessing the truth of testimony given by any interested witness?

The proper approach to assessing the truthfulness of the testimony given by any interested witness was articulated many years ago in Faryna v. Chorny, [1952] 2 D.L.R. 354, 1951 CanLII 252 (B.C.C.A.) at 357: The credibility of interested witnesses, particularly in cases of conflict of evidence, cannot be gauged solely by the test of whether the personal demeanour of the particular witness carried conviction of the truth. The test must reasonably subject his story to an examination of its consistency with the probabilities that surround the currently existing conditions. In short, the real test of the truth of the story of a witness in such a case must be its harmony with the preponderance of the probabilities which a practical and informed person would readily recognize as reasonable in that place and in those conditions.

The factors identified in Bradshaw v. Stenner, 2010 BCSC 1398 at para. 186 not only inform the assessment of whether the evidence of a witness is truthful, but also accurate or reliable.