Although credibility and reliability of a witness appear to be the same thing, they are actually quite different legal concepts.
Jung estate v Jung estate 2022 BCSC 1298 discussed the difference.
The factors to be considered when assessing credibility were summarized in Bradshaw v. Stenner, 2010 BCSC 1398 at para. 186, aff’d 2012 BCCA 296, as follows:
Credibility involves an assessment of the trustworthiness of a witness’ testimony based upon the veracity or sincerity of a witness and the accuracy of the evidence that the witness provides (Raymond v. Bosanquet (Township) (1919), 59 S.C.R. 452, 50 D.L.R. 560 (S.C.C.)). The art of assessment involves examination of various factors such as the ability and opportunity to observe events, the firmness of his memory, the ability to resist the influence of interest to lie, and the demeanour of a witness generally (Wallace v. Davis,  31 O.W.N. 202 (Ont. H.C.); Faryna v. Chorny,  2 D.L.R. 152 (B.C.C.A.) [Faryna]; R. v. S.(R.D.),  3 S.C.R. 484 at para.128 (S.C.C.)). Ultimately, the validity of the evidence depends on whether the evidence is consistent with the probabilities affecting the case as a whole and shown to be in existence at the time (Faryna at para. 356).
Credibility and reliability are separate concepts. Credibility relates to honesty whereas reliability relates to accuracy.
The starting point in a credibility assessment is to presume truthfulness; however, truthfulness and reliability are not necessarily the same: Hardychuk v. Johnstone, 2012 BCSC 1359 at para. 10.
When a witness’ evidence is clearly inaccurate “the challenge from an assessment perspective is to identify the likely reason for the inaccuracy in a cautious, balanced and contextually sensitive way”: Hardychuk at para. 10.