When will a judge ask a prospective juror about their own bias?

Second, bias is seldom overt and admitted. More often, it lies hidden and beneath the surface. An individual juror "may have an interest in concealing his own bias [or] may be unaware of it." (Smith v. Phillips (1982) 455 U.S. 209, 221, 222, 102 S.Ct. 940, 948, 71 L.Ed.2d 78 (concurring opinion by O'Connor, J.) To paraphrase an earlier statement by this court, made in the context of attorney voir dire, "[b]ecause racial, religious or ethnic prejudice or bias is a thief which steals reason and makes unavailing intelligence--and sometimes [5 Cal.App.4th 1313] even good faith efforts to be objective--trial judges [must, where appropriate, be willing to ask] prospective jurors relevant questions which are substantially likely to reveal such juror bias or prejudice, whether consciously or unconsciously held." (People v. Wells (1983) 149 Cal.App.3d 721, 727, 197 Cal.Rptr. 163.)