The twelve jurors in Twelve Angry Men have unique backgrounds and personalities. As the only one who initially votes "not guilty" and the one whose thoughtful consideration of the evidence and persuasive skills bring the entire jury over to his side, Juror 8 is an especially powerful character. If I had to guess his personality type in terms of Myers-Briggs Type Indicator, I would say he is an INTJ. You can read more about this type at the links below.
Juror 8 seems to be an introvert. As jurors enter the jury room, some are making conversation, but Juror 8 goes to stand at the window and seems lost in thought. He has to be specifically called over to join the group at the table as deliberations begin. Introverts need time alone to recharge their energy, and that is what he seems to be doing here.
The "N" means he is intuitive--he focuses on visions and possibilities. We can see that he relies not only on the face value of what he has heard, not just on his five senses, but he looks beneath the surface to imagine what might have occurred that wasn't said. That leads him to consider how long it might have taken the old man to get to the door and whether other knives like the murder weapon might be easily obtainable. Imagining the murder scene in his head allows him to join two pieces of evidence together, the passing el train and the boy shouting, to prove that the old man could not have heard the boy shout those words.
"T" means that he is a thinker--he makes decisions based on objective, logical principles. He asks to see the diagram of the room, and he re-enacts the old man's steps to show that the timing could not have worked as the man testified. Putting the two pieces of evidence together as noted in the previous paragraph shows logic as well.
"J" means he likes to have things settled and decided, which may be why he keeps working for the unanimous verdict rather than accepting suggestions that they are a hung jury.
INTJs are visionary, insightful, and like to work in a logical, orderly way. They can be obsessed with a goal. Although somewhat reserved and aloof, they are very loyal. They like to engage in intellectual discussions rather than small talk. All these characteristics seem to fit Juror 8. Interestingly, one typical occupation for INTJs is an architect, which we learn is Juror 4's profession. One of the personality sites below even nicknames this personality type "the architect."