The jurors almost unanimously vote guilty at the start of the play. While many of the jurors do simply believe the defendant is guilty of murder, other factors, such as the sweltering heat of the courthouse and the desire to get out of jury duty as soon as possible, sway them to vote as they do.
The main thing which causes most of the jurors to change their vote is Juror Eight's call for further examination of the evidence. Juror Nine changes his vote because he is impressed by Eight's courage in offering a contrarian view and wants to re-examine the evidence too.
Slowly, the other jurors start to question the evidence themselves. Five does not believe one of the witnesses, an elderly and feeble man, could so easily run out of bed to witness the crime, nor does he believe the defendant stabbed his father with a switchblade since, as a former slum-dweller himself, he knows how fights are conducted there. Nine does not believe the woman witness could see well since she was not wearing her glasses at the time of the crime. The jurors also come to re-examine their own prejudices against the defendant, particularly when Ten goes on a racist rant.
The jurors who do not take the trial seriously and just want to go home, such as Juror Twelve who is largely indifferent to the case and Juror Seven, who wants to leave so he won't miss a Broadway show (or in the movie, a baseball game), just agree with the majority so they can leave sooner. However, Juror Three is the last to hold out is Juror Three. In the play, he gives in when he sees everyone else is against him. In the movie, he realizes his insistence on voting "guilty" has more to do with his anger at his son than a genuine belief in the guilt of the defendant.