Why is the theme of justice important in Twelve Angry Men? Consider who the title Characters are, and what they do in a courtroom.
The typical jury in America is composed of 12 peers of the defendant, and their job is to determine whether the prosecution has proven beyond a reasonable doubt that the defendant has committed some wrongdoing. In the case of Twelve Angry Men, the defendant is accused of murder, and faces the death penalty if convicted, adding even more of a burden to the jury's decision, because if they convict an innocent man, he might not get another chance.
The book is how the theme of justice is shown. One Juror just happens to own a switchblade that is identical to the murder weapon. That convinces him that there is reasonable doubt. Another juror had tickets to a baseball game burning a hole in his pocket, when a storm hits, raining out the game, he lets the evidence convince him that there is reasonable doubt. Gradually other jurors also find inconsistencies in the prosecution's case that develop reasonable doubt until finally all 12 come to a verdict of 'Not Guilty.'