Witness credibility is essential to winning at trial as if a Judge does not believe your client or the witnesses then you will most certainly lose the case.
I have joked for years that the competing stories in estate dispute cases are so diverse that I caution clients that they might well initially believe they are in the wrong court room.
Mac v Mak 3026 BCSC 1140 stated:
In the often-cited Faryna v. Chorny,  2 D.L.R. 354 (B.C.C.A.), the validity of the evidence depends on whether the evidence is consistent with the probabilities affecting the case as a whole: Faryna at 357. The factors were also considered by Justice Dillon in Bradshaw v. Stenner, 2010 BCSC 1398 at para. 186:
 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 modify his recollection, whether the witness’ evidence harmonizes with independent evidence that has been accepted, whether the witness changes his testimony during direct and cross-examination, whether the witness’ testimony seems unreasonable, impossible, or unlikely, whether a witness has a motive to lie, and the demeanour of a witness generally (Wallace v. Davis (1926), 31 O.W.N. 202 (Ont.H.C.); Farnya v. Chorny,  2 D.L.R. 354 (B.C.C.A.) [Farnya]; 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 (Farnya at para. 356).