Who/what had the greatest influence on the change, and how?

I would say the 8th Juror made the most difference. He is the only juror who votes “not guilty” at the first vote. He is discontent with the way the trial was handled and wants them to discuss the evidence in greater detail. Met with much opposition, he continues to advocate for the boy. We learn that he is an architect, by trade. You can check out all the jurors at the character list below: