The movie depicted twelve men from different backgrounds and cultures. They communicated differently, as in getting their point across. This caused them to get frustrated and angry, that is where the title of the movie comes in.
I feel the movie could have been setup a lot of different ways. The one juror who voted not guilty, really didn't have a sound reason why he voted the way he did. His initial argument was the evidence was to perfect. He felt that there was no way a case could be this obvious. In my opinion he was trying to get the other jurors to look at the scenario "What if". After a while this juror brings out more valid points. He questioned the crippled man's testimony. He did not believe the man saw the young man running down the stairs as he had stated in court. The juror did a time-line experiment in the room to show the other jurors that the man's testimony did seem unrealistic. This was the beginning of many discrepancies in testimonies of witnesses.
The other jurors were not sure if the young man was guilty or innocence, but it was just easier for them to rely on their own personal feelings and bias instead of using a logical approach by analyzing the evidence they had.
Some of the jurors did not take their duty seriously enough. For example, Juror number seven was in so much of a hurry to get to the baseball game, he wanted to just vote to get it over with. It was like a task he had to complete. He did not even consider a young man's future was at stake.
The movie also shows how discrimination and bias plays a big part in decision making. A lot of the jurors were using their own life as a guide to make a comparison to this young mans life. They were making a lot of assumptions. This help distort a lot of the evidence early on in the movie because they were being stubborn to each other. Juror number seven brought that he had a previous record of petty things. This implied that since he was able to do one crime then he was capable of murder.