One of the themes of Twelve Angry Men, particularly the 1957 motion picture version, is that a person seeking to find the truth of a matter must not make assumptions, but must take a fresh perspective by asking questions. Most of the jurors make multiple assumptions. Juror 8 is the one who distinguishes himself by questioning some assumptions the other jurors hold.
At the beginning, most of the jurors are under the assumption that the prosecuting attorney and the defense attorney were consummate professionals and had provided clear cut evidence in the case, leaving no stone unturned. This turns out to be a false assumption, for the jurors are able to make many discoveries that were not brought up during the trial. All the men except Juror 8 assume the truth about the knife found at the scene, that it had a unique design. Juror 8 asked himself whether that was true; he went out the previous night and was able to purchase an identical knife just blocks from where the defendant lived.
Most of the jurors begin deliberations by assuming the two primary witnesses in the case gave accurate testimony. By asking questions, Juror 8 was able to cast doubt upon the stories told by the old man and the "eyewitness." All the jurors, even Juror 8, assume that the entry wound was consistent with the murder weapon because they were told that in the trial. Only when Juror 5 remembers seeing knife fights in his youth does he demonstrate that the wound could not have been made by a person who routinely used a switchblade. The drama shows that by asking questions and shedding their assumptions, the men were able to get a clearer picture of the truth. In this case, it was important enough to keep an innocent boy from the electric chair, but asking questions and avoiding assumptions is important any time one wants to bring clarity to a situation. It is the way to move out stalled thinking toward a positive solution.