You need to flip a coin, heads means you're a guard and tails makes you a prisoner.
ARREST OF PRISONERS
You're under arrest!
Zimbardo randomly selected the participants roles by putting them in pairs and making them flip a coin, this eliminated any bias and made the selection totally random.
The police arrested the participants that were assigned the role of prisoner at their houses (mundane realism). The participants were unaware this would happen and so were shocked.
Prisoners were strip-searched and deloused to make it seem more realistic. They were made to wear a uniform (that barley covered them) and were not allowed to wear underwear. They had to wear an ankle chain and could only refer to each other by their prison numbers.
Prisoners were forced to do press-ups as punishment and some were put into solitary confinement. The guards became more brutal and refused to let prisoners leave the cells. One prisoner showed extreme anxiety but was not allowed to leave until day 4.
Please let me leave
The guards made prisoners clean toilets with their bare hands and at night they would sexually humiliate prisoners.
Please don't make me do it!
The guards power was almost total and prisoners obeyed without question. Christina Maslach (Zimbardo's girlfriend) visited the 'prison' and strongly objected to it. She forced Zimbardo to shut it down by threatening to break up with him.