Cal. CCP § 234

Whenever, in the opinion of a judge of a superior court about to try a civil or criminal action or proceeding, the trial is likely to be a protracted one, or upon stipulation of the parties, 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.”

These alternate jurors shall be drawn from the same source, and in the same manner, and have the same qualifications, as the jurors already sworn, and shall be subject to the same examination and challenges. However, each side, or each defendant, as provided in Section 231, shall be entitled to as many peremptory challenges to the alternate jurors as there are alternate jurors called.

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 shall, unless excused by the court, attend at all times upon the trial of the cause in company with the other jurors, but shall not participate in deliberation unless ordered by the court, and for a failure to do so 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 who shall not suffer any communication to be made to them except by order of the court, and shall not be discharged until the original jurors are discharged, except as provided in this section.

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 his or her place in the jury box, and be subject to the same rules and regulations as though he or she had been selected as one of the original jurors.

All laws relative to fees, expenses, and mileage or transportation of jurors shall be applicable to alternate jurors, except that in civil cases the sums for fees and mileage or transportation need not be deposited until the judge directs alternate jurors to be impaneled.