By act 3 of Twelve Angry Men, tensions have risen in the tiny jury room. Despite several votes, the 12 men cannot agree on the guilt of the defendant. Reginald Rose constantly has characters refer to the heat in the room and the exasperation as they cannot come to an agreement.
In this part of act 3, Juror #2 (the characters are only known by the juror numbers or position in the court) questions if the boy would've been able to stab his father in the manner suggested by the prosecution. His points out that the boy is shorter than his father and, therefore, could not have easily stabbed his taller father:
NO. 2: I know they did—but I don't go along with it. The boy is five feet eight inches tall. His father was six two. That's a difference of six inches. It's a very awkward thing to stab down into the chest of someone who's half a foot taller than you are.
Exasperated that Juror #2, who hasn't really spoken up until this point, has raised a question about the boy's guilt, Juror #3 jumps out of his seat, flicks open a pocket knife and proposes he "gives [Juror #2] a demonstration."
Juror #2 yells, "Look out!" as #3 emulates a stabbing motion stopping just inches of Juror #8's chest.
Because of the tension in the room, the men are not positive that Juror #3 is not going to stab #8. The scene demonstrates how the men are frustrated and that several of them are becoming unhinged during the discussions. Juror #3 also successfully demonstrates how someone can stab another taller person, continuing the debate on whether or not the boy really did kill his father.