“Conclave” purports to unveil how the College of Cardinals elects a new Pope for the Roman Catholic Church.

Fiction with thriller elements, based on Robert Harris’ bestselling novel also called “Conclave,” it stars Ralph Fiennes as Cardinal Lawrence, tasked with overseeing the voting, and Isabella Rossellini as Sister Agnes, a nun who cannot, of course, vote but is responsible for the cardinals’ care while they are in Rome.

“We know, first of all, very little about the voting because obviously the door is closed and it’s the most secretive election in the world,” director Edward Berger acknowledged during a virtual press conference in Rome, not far from where they filmed in the legendary Cinecittà studios.

“To take us behind those closed doors we had a lot of great religious advisors. A theologist was there every day and told us about what is known about the selections. There was a wealth of material to fall back on.”

“Our religious advisor on set was definitely very informed,” Fiennes, 61, agreed. “Quite a bit is known about the actual system of voting. That’s all documented. None of us knows about the conversations that go in the Casa Santa Marta, which is where all the cardinals stay. All that stuff is speculation.

“I was brought up Catholic when I was young,” he continued. “I went to Catholic primary school in the UK and the Catholic boys’ school in Ireland. So I had a sense of the masculine hierarchical structure of much of the Catholic church and felt I had a little foothold in some aspects of it.”

“I was born in Rome, grew up in Rome, though I’ve never been part of a conclave. Not yet,” Rossellini, 72, said to laughter. “I was a bit more familiar than some, at least with the look of the Sistine Chapel, the ritual, the authority, the secrecy, and the respect of the secrecy.

“I went to a nun school. That was very good because I knew that these nuns, they don’t speak, but they are very severe. They know there was a very specific hierarchy. But these women were not submissive. They were silent — but not submissive. That was very clear to me.”

Fiennes’ Cardinal Lawrence is overseeing but not competing for votes to become Pope. “It’s clear quite early on,” Fiennes said, “he’s a reluctant manager and has considered leaving the Vatican. Maybe pursuing a more monastic life.

“He is a man of integrity and reminds me of George Smiley in John Le Carré’s books, a sideways investigator. I’ve met some cardinals and believe they are of deep spirituality.

“Lawrence has a kind of wisdom but we are all fallible. None of us are saints.”

“Conclave” opens in theaters Oct. 25