What starts the physical confrontation between two jurors? and which jurors?

After a brief argument, 8th Juror brings into question whether or not the downstairs neighbor, an old man who had suffered a stroke and could only walk slowly, could have gotten to the door to see the boy run down the stairs in fifteen seconds, as he had testified. 8th Juror recreates the floor plan of the apartment, while 2nd Juror times him, and they conclude that he would not have been able to reach his door in fifteen seconds.

3rd Juror reacts violently to this, calling it dishonest, saying that this kid had “got to burn.” 8th Juror calls him a “self-appointed public avenger” and a “sadist,” and 3rd leaps at him. Restrained by the other men, he shouts, “God damn it! I’ll kill him! I’ll kill him.” 8th Juror asks, “You don’t really mean you’ll kill me, do you?” proving his earlier point about how people say, “I’ll kill you,” when they don’t really mean it.