Lee J. Cobb
Nothing is shown from the trial itself; the audience is merely introduced to the twelve residing jury members as they talk amongst themselves and dish out the facts of the case, breaking down each and every element one by one. It's a mystery where the truth is never revealed, because, as is such with life itself, there's no way to be sure, and the characters are forced to reach their verdict on the facts presented and if those facts justify putting a young man to death. The main lesson here, it would seem, is built around two ideals: people should always be treated as innocent until proven guilty (meaning it's the job of the prosecutor to prove the accused party guilty, not the job of the accused to prove their innocence), and to make sure the accused party is guilty beyond reasonable doubt before reaching a verdict declaring such.
However, I believe the film's ending inadvertently proves why the justice system is so flawed (and don't worry, this can only be considered a spoiler if you're horrible at predicting endings): the verdict the jury reaches is nonsensical, as there is not reasonable doubt to exonerate the boy from the murder. Doubt, certainly... but not reasonable doubt. Henry Fonda's character is treated like a hero by the end of the film, as he rallies up other members of the jury to join him in his picking apart of all the facts in the case, and showing how each element may be flawed. The key word though is "may," and considering that there would have to be about a half dozen horribly unfortunate coincidences to make they boy seem guilty when in reality he were innocent, the audience is left with the creeping realization that a murderer has now been let free. In all honesty though, this makes the movie even more effective for me as a viewer... just probably not in the way it had originally been intended.
Audio Commentary (with film historian Drew Casper): Casper is one of USC's professors of Critical Studies in the School of Cinematic Arts, and though he has plenty of info to offer, I would have much rather been offered a commentary from the film's director, Sidney Lumet.
Beyond A Reasonable Doubt: Making 12 Angry Men (23:03): Your standard interview-filled featurette, including sessions with director Sidney Lumet and the sole juror from the film still living, Jack Klugman.
Inside the Jury Room (15:28): A featurette that explores the accuracy of the film.