Montana Title 26. Evidence MT R REV Rule 606.Competency of juror as witness

(a) At the trial. A member of the jury may not be called or testify as a witness before that jury in the trial of the case in which the juror is sitting. If the juror is called to testify, the opposing party shall be afforded an opportunity to object out of the presence of the jury.

(b) Inquiry into validity of verdict or indictment. 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 as influencing the juror to assent or dissent from the verdict or indictment or concerning the juror's mental processes in connection therewith. Nor may a juror's affidavit or evidence of any statement by the juror concerning a matter about which the juror would be precluded from testifying be received for these purposes.

However, as an exception to this subdivision, a juror may testify and an affidavit or evidence of any kind be received as to any matter or statement concerning only the following questions, whether occurring during the course of the jury's deliberations or not: (1) whether extraneous prejudicial information was improperly brought to the jury's attention; or (2) whether any outside influence was brought to bear upon any juror; or (3) whether any juror has been induced to assent to any general or special verdict, or finding on any question submitted to them by the court, by a resort to the determination of chance.