In the end, you will have to select the most pertinent details from the trial and write about your impressions. Placing yourself there is what makes this such a great assignment. There cannot be that much in way of wrong answers. You could focus on how the atmosphere of the trial was one in which this case "was one of the strangest" on record. You could talk about how the defendant was "an ugly customer" and how everyone in the courtroom presumed him to be guilty. Another part of this account could involve how Mrs. Salmon was an impeccable witness in her approach to being on the witness stand as she approached it. Addressing the atmosphere of certainty as she delivered her testimony might be effective in the account, especially given how "it was all over but the hanging."
Another element of your account could be the sudden shift of certainty when the "other" Mr. Adams stands up. Part of this account could involve the way Mrs. Salmon shook her head when it became clear that she was mistaken. These aspects could figure into your account as you detail the fluid shift between what was perceived to be certain and absolute guilt to what actually came about. In your account, placing yourself there as this dynamic becomes a part of this account could be quite persuasive in reclaiming the mood that Greene establishes throughout the story.