What is the current state of the law on credibility of witnesses in a witness trial?

As defendant points out, "'[i]f a party introduces evidence which tends to impeach a witness of his opponent, the latter may in rebuttal offer evidence to support his witness' credibility. [Citation.]'" (People v. Adamson (1953) 118 Cal.App.2d 714, 719.) In other words, evidence to support the witness's credibility would then be relevant. On

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the other hand, "[a] fact may bear on the credibility of a witness and still be collateral to the case. [Citation.]" (People v. Contreras, supra, 58 Cal.4th at p. 152.)