California Code, Evidence Code - EVID § 704

(a) Before a juror sworn and impaneled in the trial of an action may be called to testify before the jury in that trial as a witness, he shall, in proceedings conducted by the court out of the presence and hearing of the remaining jurors, inform the parties of the information he has concerning any fact or matter about which he will be called to testify.

(b) Against the objection of a party, a juror sworn and impaneled in the trial of an action may not testify before the jury in that trial as a witness. Upon such objection, the court shall declare a mistrial and order the action assigned for trial before another jury.

(c) The calling of a juror to testify before the jury as a witness shall be deemed a consent to the granting of a motion for mistrial, and an objection to such calling of a juror shall be deemed a motion for mistrial.

(d) In the absence of objection by a party, a juror sworn and impaneled in the trial of an action may be compelled to testify in that trial as a witness.