In many legal systems, jurors are expected to base their decisions solely on the evidence presented during the trial. However, if a potential juror has prior knowledge of the case or has formed an opinion about the guilt or innocence of the defendant, they may be disqualified from serving on the jury. This is because impartiality and fairness are essential principles in the jury system.

During jury selection, known as voir dire, potential jurors are typically asked questions by the judge and attorneys to determine if they have any biases or conflicts of interest that could affect their ability to render an impartial verdict. If a potential juror discloses that they have prior knowledge of the case or have formed an opinion about it, they may be dismissed from jury duty.

In some cases, jurors may be allowed to serve if they can convincingly demonstrate that they can set aside their preconceptions and judge the case solely on the evidence presented in court. However, the decision ultimately lies with the judge and attorneys involved in the case.