What is the relevant case law on substantial evidence review?

As with a substantial evidence review, we do not reweigh the evidence in applying the demonstrable reality test. (Barnwell, supra, 41 Cal.4th at p. 1053.) We afford deference to the trial court's credibility determinations and factual determinations, as they are based on firsthand observations. (Ibid.) "[N]ot every incident involving a juror's conduct requires or warrants further investigation. 'The decision whether to investigate the possibility of juror bias, incompetence, or misconductlike the ultimate decision to retain or discharge a jurorrests within the sound discretion of the trial court.' " (People v. Cleveland (2001) 25 Cal.4th 466, 478.) "[A] trial court's inquiry into possible grounds for discharge of a deliberating juror should be as limited in scope as possible, to avoid intruding unnecessarily upon the sanctity of the jury's deliberations. The inquiry should focus upon the conduct of the jurors, rather than upon the content of the deliberations." (Id. at p. 485.)