Tex. R. Evid. 606

As amended through August 21, 2020
Rule 606 - Juror's Competency as a Witness
(a) At the Trial. A juror may not testify as a witness before the other jurors at the trial. If a juror is called to testify, the court must give a party an opportunity to object outside the jury's presence.
(b) During an Inquiry into the Validity of a Verdict or Indictment.
(1)Prohibited Testimony or Other Evidence. During an inquiry into the validity of a verdict or indictment, a juror may not testify about any statement made or incident that occurred during the jury's deliberations; the effect of anything on that juror's or another juror's vote; or any juror's mental processes concerning the verdict or indictment. The court may not receive a juror's affidavit or evidence of a juror's statement on these matters.
(2)Exceptions. A juror may testify:
(A) about whether an outside influence was improperly brought to bear on any juror; or
(B) to rebut a claim that the juror was not qualified to serve.

Tex. R. Evid. 606