For actor Billy Crudup, starring in “The Stanford Prison Experiment” (opening Friday) had a more profound impact than he thought it would after first reading the film’s screenplay.

“I sort of grew up with the notion that most institutions had been in place for hundreds if not thousands of years. Whether it was the judicial system, financial system, our prison system — plus government and educational systems — I kind of thought of them being in place and handed down from on high,” said “Almost Famous” and “Watchmen” actor, calling to chat about the new movie.

“Making this film made me really stop and remember that all of these institutions were created by people, and can be changed by people — if they so desire to do so. I made me realized there’s a mandate on us as citizens to consider the way our country and our institutions within that country operate.

“Hopefully, the consciousness of that power in a democratic society that comes with that knowledge will help inform me — and everyone else — to be a better citizen. I certainly found it a rewarding thing to be reminded of,” said Crudup.

In “The Stanford Prison Experiment,” Crudup plays Dr. Philip Zimbardo, a psychology professor at the famed California university who embarked upon an experiment in the summer of 1971 to discover the effect prison life could have on individuals — even after a relatively short period of time.

Unbeknownst to Zimbardo, that time frame would be shortened far more than he could have ever imagined when he was developing the details. He and his Stanford colleagues recruited 24 paid student volunteers and split them evenly into a group of prisoners and guards. They were then placed in a faux prison built on the school’s campus — empty for the summer months.

After only a few days, instead of the originally planned two weeks, the experiment had to be aborted when the environment led the middle-class undergraduates playing the guards to become abusive, sadistic power-drunk hellions who virtually had their cruel way with the dozen “prisoners” they controlled.

Zimbardo and his associates were equally stunned — and horrified — at how quickly the “prisoners” allowed themselves to become totally submissive and even willing victims of the “guards.”

Considering all the recent news about alleged abuse and even fatal actions taken by police figures in positions of authority, it seemed relevent to ask Crudup about the timeliness of “The Stanford Prison Experiment.”

The actor turned pensive as he said, “Unfortunately, my sense is that this film will always be timely, because there’s an aspect of human nature that is wrapped up in vigorously fighting to protect their role in a hierarchical structure. As social beings, we constantly need to be on guard to protect ourselves and society from an authority system that overreaches.”

The actor was pleased he got to meet the real Zimbardo, who remains affiliated with Stanford.

“He came to set one day, which was both fascinating and intimidating to be sure. He’s a very formidable person, but also very affectionate and generous. It was great to have him around, and he was thrilled to be sharing this very complicated and confusing experience with us.”

When asked what specific advice Zimbardo gave to help portray him more accurately, Crudup laughed. “It was funny. The only suggestion he had for me was, ‘Billy, I’m Italian. You have to use your hands more!’ ”

Beyond that, Crudup said he really didn’t need to spend long hours in conversation with Zimbardo. “He had been working with one of our producers, Brent Emery, and the screenwriter, Tim Talbott, for a very long time. Dr. Zimbardo’s work is in the screenplay. He didn’t have to tell me anything beyond that.”

Yet Crudup admitted he did find something extremely useful as he did a bit of research beyond merely reading the screenplay. “I discovered how painful and upsetting it was to Dr. Zimbardo to realize he had caused pain and upset to a number of people [who participated in the experiment] because he had lost a piece of himself as an academic in the process. By losing himself in the experiment he made a big mistake. That led to him learning a degree of humility, and that was impressive to witness.

“That ultimate humility did become part of my performance.”