What is relevant evidence that a witness is afraid to testify?

Only relevant evidence is admissible. ( 350.) Relevant evidence includes "evidence relevant to the credibility of a witness." ( 210.) Section 1101, subdivision (a), which limits the admissibility of character evidence, does not "affect[] the admissibility of evidence offered to support or attack the credibility of a witness." (Id., subd. (c).) "Evidence that a witness is afraid to testify or fears retaliation for testifying is relevant to the credibility of that witness and is therefore admissible. [Citations.] An explanation of the basis for the witness's fear is likewise relevant to her credibility and is well within the discretion of the trial court. [Citations.]" (People v. Burgener (2003) 29 Cal.4th 833, 869.) "A witness who testifies despite fear of [retaliation] of any kind by anyone is more credible because of his or her personal stake in the testimony. . . . [] [T]he jury would be entitled to . . . know not just that the witness was afraid, but also, within the limits of Evidence Code section 352, those facts which would enable them to evaluate the witness's fear." (People v. Olguin (1994) 31 Cal.App.4th 1355, 1368-1369.)