Cal. Penal Code § 1120

If a juror has any personal knowledge respecting a fact in controversy in a cause, he or she must declare the same in open court during the trial. If, during the retirement of the jury, a juror declares a fact that could be evidence in the cause, as of his or her own knowledge, the jury must return into court. In either of these cases, the juror making the statement must be sworn as a witness and examined in the presence of the parties in order that the court may determine whether good cause exists for his or her discharge as a juror.