Generally, three considerations guide the receivability of evidence:
Relevant: Evidence must be relevant in the sense of being probative of a fact a party seeks to establish.
Material: Evidence must also be material in the sense that the fact the evidence is proffered to establish relates to an issue in the case.
Admissible: Finally, the evidence must not contravene any admissibility rule. If one of the requirements is not satisfied, the evidence must not be admitted.
For a detailed summary of these three factors, see Watt J.A.’s analysis in R. v. Luciano, 2011 ONCA 89 (CanLII),273 O.A.C. 273, at paras. 203-211.
The Admissibility of Prior Consistent Statements
It is impermissible to assume that because a witness has made the same statement in the past, he or she is more likely to be telling the truth, and any admitted prior consistent statements should not be assessed for the truth of their contents.
As a result, prior consistent statements are not admissible as evidence for the truth of their contents. Such statements lack probative value, are often self-serving, and are hearsay.
R. v. Stirling, 2008 SCC 10 (CanLII),  1 S.C.R. 272, at para 5; R. v. Dinardo, 2008 SCC 24(CanLII),  1 S.C.R. 788, at para. 36.
Prior consistent statements lack probative value as, by definition, they are merely a repeat of evidence. Their lack of probative value stems from the fact that it is impermissible to assume that because a witness has made the same statement in the past, he or she is more likely to be telling the truth. Thus, repetition does not demonstrate or prove anything. In R. v. Divitaris (2004), 2004 CanLII 9212 (ON CA), 188 C.C.C. (3d) 390 (Ont. C.A.), at para. 28, Feldman J.A. memorably expressed the rationale for the rule in this way: “A concocted statement, repeated on more than one occasion, remains concocted”.
Demeanor Evidence and the Inadmissible Prior Consistent Statement
If the content of the prior consistent statement is inadmissible for its truth, it follows that evidence of the witness’s demeanor in giving that prior consistent statement is likewise inadmissible if the sole purpose of that demeanor evidence is to invite the trier-of-fact to conclude that the prior statements were both truthful and consistent with the witnesses sworn evidence at trial.
See for instance R. v. Rhayel, 2015 ONCA 377 (CanLII)