Since you put this in the To Kill a Mockingbird group, I assume you are referring to what happens in this book.  To me, the reason that jury duty in the Tom Robinson case is difficult is that it forces the ordinary person to balance what they think about race relations with the evidence that they see in the case.

An ordinary white juror in this case would have felt a lot of pressure to convict Robinson.  They would have assumed that he was guilty because they all expected black men to prey on white women when given the chance.  They expected blacks to lie as well.

But then they had to listen to the evidence in the case and notice that it was Ewell who seemed to be lying.

Perhaps this is why it took them so much longer than expected to come to a verdict in this case.