Cal. Pen. Code § 1089
Whenever, in the opinion of a judge of a superior court about to try a defendant against whom has been filed any indictment or information or complaint, the trial is likely to be a protracted one, the court may cause an entry to that effect to be made in the minutes of the court, and thereupon, immediately after the jury is impaneled and sworn, the court may direct the calling of one or more additional jurors, in its discretion, to be known as "alternate jurors."
The alternate jurors must be drawn from the same source, and in the same manner, and have the same qualifications as the jurors already sworn, and be subject to the same examination and challenges, provided that the prosecution and the defendant shall each be entitled to as many peremptory challenges to the alternate jurors as there are alternate jurors called. When two or more defendants are tried jointly each defendant shall be entitled to as many peremptory challenges to the alternate jurors as there are alternate jurors called. The prosecution shall be entitled to additional peremptory challenges equal to the number of all the additional separate challenges allowed the defendant or defendants to the alternate jurors.
The alternate jurors shall be seated so as to have equal power and facilities for seeing and hearing the proceedings in the case, and shall take the same oath as the jurors already selected, and must attend at all times upon the trial of the cause in company with the other jurors, and for a failure so to do are liable to be punished for contempt.
They shall obey the orders of and be bound by the admonition of the court, upon each adjournment of the court; but if the regular jurors are ordered to be kept in the custody of the sheriff or marshal during the trial of the cause, the alternate jurors shall also be kept in confinement with the other jurors; and upon final submission of the case to the jury the alternate jurors shall be kept in the custody of the sheriff or marshal and shall not be discharged until the original jurors are discharged, except as hereinafter provided.
If at any time, whether before or after the final submission of the case to the jury, a juror dies or becomes ill, or upon other good cause shown to the court is found to be unable to perform his or her duty, or if a juror requests a discharge and good cause appears therefor, the court may order the juror to be discharged and draw the name of an alternate, who shall then take a place in the jury box, and be subject to the same rules and regulations as though the alternate juror had been selected as one of the original jurors.
Ca. Pen. Code § 1089