During a trial, a member of the jury may not testify as a witness before that jury in the trial of the case in which the juror is sitting. If the juror is called so to testify, the opposing party will be afforded an opportunity to object out of the presence of the jury.
Similarly, upon an inquiry into the validity of a verdict or indictment, a juror may not testify as to any matter or statement occurring during the course of the jury's deliberations or to the effect of anything upon that or any other juror's mind or emotions influencing the juror to assent to or dissent from the verdict.
However, a juror can testify about whether extraneous prejudicial information was improperly brought to the jury's attention, whether any outside influence was improperly brought to bear upon any juror, or whether there was a mistake in entering the verdict onto the verdict form. [USCS Fed Rules Evid R 606]