During filming in Rome, the crew and equipment were blocking the passage of a bridal party on its way to the church for a wedding. Upon discovering this situation, Tom Hanks personally escorted the party through the filming area and rallied the crew to remove the obstacles in the way. After the bride was delivered to the church (on time), her grateful family invited Hanks and director Ron Howard to stay for the wedding ceremony. Unfortunately, Hanks and Howard's busy filming schedule prevented them from accepting the offer.
Ewan McGregor suggested to Ron Howard that he could do the sequence where the Carlamengo tells the cardinals about the Illuminati entirely without breaking it into several segments as felt that he could pull it off having known all of his lines there. Howard agreed to his suggestion and proceeded to film it. The take was very good, much to the applause of everyone that it was the only one shot and used in the final film.
Crew members visited Vatican City as tourists and extensively photographed the city to capture as much detail as possible, knowing they were unlikely to be allowed to film there, so that they could recreate the sets as faithfully as possible.
In the previous film, Langdon remarks to Sophie Neuveu that perhaps she cured his claustrophobia by laying her hands upon him. It appears that she did so indeed, as in this movie Langdon is twice confined in a small, glass room with poor ventilation and seems un-fazed. He also seems fine when he needs to squeeze through a small alley between two very close walls in his search for the bomb.
The 'God particle' referred to in the movie is the Higgs-Boson particle, theorized in 1963-64. The LHC in Geneva is indeed seeking out the nature of this particle, which is claimed to give mass to all matter in the Universe.
When Langdon is in his office speaking with the Vatican employee, you can see a "Phillips Exeter Academy" flag on the wall. Phillips Exeter is the school where Angels & Demons author Dan Brown taught and once attended, located in New Hampshire, and where Langdon is written to have attended in the novels.
The final piece before the end credits is Dr. Morten Lauridsen's "Lux Aeterna: Introitus" - Dr. Lauridsen has never before allowed this piece to be used in a movie before due to the context being inappropriate to his meaning behind the piece.
This movie became the first big-screen casualty of the Hollywood writer's strike in 2007, because Akiva Goldsman's script still needed work, and he was on strike with the Writers Guild of America. So, production of the movie had to be postponed.
As well as providing the voice over for the film's teaser trailer (using an American accent) Alfred Molina (who played Aringarosa in the previous movie The Da Vinci Code (2006)) provides the opening voice over for Angels & Demons (2009), this time in his native English accent.
Many of the sets were a combination of physical set pieces and green screens with the backgrounds to be digitally added later. This technique was used for scenes that the crew was unable to film at either due to the volume of tourists, or because the Church denied them permission.
As with the other two films in the trilogy, author Dan Brown named main character Robert Langdon after John Langdon, a close friend and typography master who worked with Brown on ambigram designs for his book, "Angels And Demons", as well as the films. John Langdon also designed an ambigram that was used in the movie Monkeyshine (2008).