Although never mentioned in the movie, the real life experiment was funded by the U.S. Office of Naval Research and was of interest to both the U.S. Navy and Marine Corps as an investigation into the causes of conflict between military guards and prisoners.
The $15 a day all the students were offered would equal $94.11 a day in 2016 if changed for inflation. That is $1317.54 in 2 weeks. In the real life experiment in 1971 the students were paid $20 a day which is $125.48 in 2016 dollars or $1756.72 in 2 weeks.
During the real experiment the prisoners were "arrested" at their homes and "charged" with armed robbery. The local Palo Alto police department assisted Zimbardo with the arrests and conducted full booking procedures on the prisoners, which included fingerprinting and taking mug shots. The prisoners were then transported to the mock prison from the police station, where they were strip searched and given their new identities.
Dr. Zimbardo had all of the "prison guards" wear mirrored sunglasses as a part of their uniforms. This look was inspired by the sinister Boss Godfrey from Cool Hand Luke. Zimbardo believed the mirrored sunglasses would further separate the "prisoners" from the "guards," and add apathy & coldness to their appearance.
Shot in 21 days.