The undercover operation enabled producers to faithfully reproduce the Holy See, where much of the action in the film takes place.
The thriller, which has its world premiere in Rome on May 4, stars Tom Hanks as Harvard symbologist Robert Langdon racing against time to unravel a plot to destroy the Vatican.
The Catholic Church refused to let the movie be filmed in the Vatican or in any of its churches in Rome because of its anger over The Da Vinci Code, which revolves around the idea that Jesus married Mary Magdalene and secretly fathered children. The ban included the filming of church exteriors.
The Vatican said the film, which also stars Ewan McGregor and was adapted from Brown's best-selling book, was "an offence against God".
But that left producers with a big problem.
"The ban on filming put us in serious difficulty because we were not able to carry out the photographic surveys necessary to reconstruct the setting," special effects supervisor, Ryan Cook, told an Italian film magazine.
"So for weeks we sent a team of people who mixed with tourists and took thousands of photos and video footage."
The photos and film helped digital effects specialists recreate computer-generated images of the imposing statues, colonnades and monuments which encircle St Peter's Square, right down to the shadows they cast on the ground.
"We filled the square with a huge crowd, created mostly with digital figures, some standing still, others moving, clapping their hands or scratching their noses," Mr Cook told the magazine Ciak.
Ron Howard, the director, also had a scale model of the Vatican built in a film studio in Los Angeles in order to get around the Vatican's ban on filming.
He hinted in an interview in December that he had used a spot of subterfuge to complete the film, which is a prequel to The Da Vinci Code.
"We didn't shoot at the Vatican officially. But cameras can be made really small," he told an American television programme, Shootout.
The challenge of filming interior shots was overcome by using the grandiose halls and sweeping staircases of a former royal palace at Caserta, near Naples.
Angels and Demons, which Brown wrote before The Da Vinci Code, follows Langdon as he tries to stop a suspected plot by the Illuminati, an ancient secret society, from destroying the Vatican.
In announcing its prohibition on filming last year, Archbishop Velasio De Paolis, head of the Vatican's Prefecture for Economic Affairs, said Brown's books were blasphemous and "turned the Gospels upside down." Father Marco Fibbi, spokesman for the diocese of Rome, added: "Normally we read the script, but this time it was not necessary. The name Dan Brown was enough."
But the Vatican will have to brace itself for fresh indignation – Brown's long-awaited follow-up book, The Lost Symbol, will be released in September.
It has an initial print run of five million copies and explores the origins of Freemasonry.
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