Demeanour. R v Rhayel
In R. v. Rhayel (Ont CA, 2015) the Court of Appeal cautioned against undue reliance by the trier of fact on demeanour evidence, as follows:
 Cases in which demeanour evidence has been relied upon reflect a growing understanding of the fallibility of evaluating credibility based on the demeanour of witnesses: see Law Society of Upper Canada v. Neinstein (2010), 2010 ONCA 193 (CanLII), 99 O.R. (3d) 1 (C.A.), at para. 66; R. v. G. (P.), 2012 ONSC 4646 (CanLII), 104 W.C.B. (2d) 390, at paras. 31-33; 9129-9321 Quebec Inc. v. R., 2007 TCC 2 (CanLII),  T.C.J. No. 23, at para. 31; R. v. Powell,  O.J. No. 555, at paras. 9-10. It is now acknowledged that demeanour is of limited value because it can be affected by many factors including the culture of the witness, stereotypical attitudes, and the artificiality of and pressures associated with a courtroom. One of the dangers is that sincerity can be and often is misinterpreted as indicating truthfulness.
 In R. v. G.(M.) (1994), 1994 CanLII 8733 (ON CA), 93 C.C.C. (3d) 347 (C.A.), at p. 355, this court quoted with approval the following passage from Faryna v. Chorny,  2 D.L.R. 354 (B.C.C.A.), at pp. 356-57:
If a trial judge's finding of credibility is to depend solely on which person he thinks made the better appearance of sincerity in the witness box, we are left with a purely arbitrary finding and justice would then depend upon the best actors in the witness box. On reflection it becomes almost axiomatic that the appearance of telling the truth is but one of the elements that enter into the credibility of the evidence of a witness. This cautious approach to demeanour was echoed by the Alberta Court of Appeal in R. v. Pelletier (1995), 1995 ABCA 128 (CanLII), 165 A.R. 138, at para. 18, citing a 1973 paper by Justice MacKenna:
For a trial judge to say "I believe him because I judge him to be telling the truth," is to come to a conclusion on consideration of only half the problem. In truth it may easily be self-direction of a dangerous kind.
I question whether the respect given to our findings of fact based on the demeanour of the witnesses is always deserved. I doubt my own ability, and sometimes that of other judges, to discern from a witness’s demeanour, or the tone of his voice, whether he is telling the truth. He speaks hesitantly. Is it the mark of a cautious man, whose statements are for that reason to be respected, or is he taking time to fabricate? Is the emphatic witness putting on an act to deceive me, or is he speaking from the fullness of his heart, knowing that he is right? Is he likely to be more truthful if he looks me straight in the face than if he casts his eyes on the ground, perhaps from shyness or a natural timidity? For my part I rely on these considerations as little as I can help. A powerfully-worded and thoroughly-researched analysis of reliance on demeanour evidence can be found in the decision of the High Court of Australia in State Rail Authority of New South Wales v. Earthline Constructions Pty. Ltd. (1999), 160 A.L.R. 588. At para. 88, the Court says:
There is growing understanding, both by trial judges and appellate courts, of the fallibility of judicial evaluation of credibility from the appearance and demeanour of witnesses in the somewhat artificial and sometimes stressful circumstances of the courtroom. Scepticism about the supposed judicial capacity in deciding credibility from the appearance and demeanour of a witness is not new. In Societe D’Avances Commerciales (Societe Anonyme Egyptienne) v Merchants’ Marine Insurance Co (The “Palitana”), Atkin LJ remarked that “an ounce of intrinsic merit or demerit in the evidence, that is to say, the value of the comparison of evidence with known facts, is worth pounds of demeanour.” To some extent, the faith in the judicial power to discern credibility from appearance was probably, at first, a consideration which the judiciary assumed that it inherited from juries. It was natural enough that trial judges, accustomed to presiding over jury trials, would claim, and appellate judges would accord, the same “infallible” capacity to tell truth from falsehood as had historically been attributed to the jury. Nowadays, most judges are aware of the scientific studies which cast doubt on the correctness of this assumption. I agree with the suggestion contained at the conclusion of the Court’s analysis in the State Rail Authority decision that it is important for trial judges to bear in mind that, to the extent possible, they should try to decide cases that require assessing credibility without undue reliance on such fallible considerations as demeanour evidence.