CALIFORNIA EVIDENCE: CIVIL AND CRIMINAL
...Competency of Witnesses
......Juror as Witness
4 Cards On This Topic:

Before testifying, juror must first inform parties, outside presence of other jurors, of information upon which testimony is based.

If party objects, court shall declare mistrial if one side calls sitting juror to testify. Objection to calling of sitting juror as witness shall be deemed motion for mistrial.

Competency of juror as witness.

That a juror excused during trial for having overheard D calling W was later allowed to testify on the subject as DA's witness violated D's constitutional due process right to a fair trial.