What is the test for assessing a prospective juror's true state of mind?

after examining the available evidence, which typically includes the juror's written responses in a jury questionnaire and answers during voir dire, the trial court need only be left with a definite impression that the prospective juror is unable or unwilling to faithfully and impartially follow the law. [Citations.] [] Second, in assessing a prospective juror's true state of mind, the trial court occupies a superior position vis- -vis an appellate court, for the former court is able to consider and evaluate a juror's demeanor during voir dire. [Citations.] ' " '[A]ppellate courts recognize that a trial judge who observes and speaks with a prospective juror and hears that person's responses (noting, among other things, the person's tone of voice, apparent level of confidence, and demeanor) ... gleans valuable information that simply does not appear on the record.' " [Citations.]' [Citation.] Accordingly, the trial court's ruling regarding the juror's true state of mind is entitled to deference on appeal if supported by substantial evidence. [Citations.]" ( People v. Thompson , supra , 1 Cal.5th at p. 1066, 210 Cal.Rptr.3d 667, 384 P.3d 693.)