While researching the life and work of Fr. Gabriel Amorth, Russell Crowe discovered that Amorth's personal favorite movie was The Exorcist (1973), so much so, that Amorth became good friends with its director William Friedkin who later directed a documentary of Amorth's work as an exorcist, The Devil and Father Amorth (2017). This would be the second to last film Friedkin directed to be released in his lifetime before his death on August 7, 2023.
Russell Crowe came up with the idea of Gabriele Amorth riding a Lambretta. He was inspired by seeing priests driving through Rome on Lambrettas and wanted to include it in the film.
Russell Crowe met with personal friends of Fr. Amorth including one who had accompanied Amorth on over 150 exorcisms, in order to better know the character he was playing.
According to an interview with Daniel Zovatto, in his research, he found out that Fr. Amorth would often keep a sense of humor when conducting exorcisms because though the devil enjoyed irony, he hated humor. The example of demons liking irony is prominent in the scene where the possessed child causes the cross to fall off the wall saying, "Did (God) allow that?" and the demon's disgust for humor is noticeable in the next moment where the child says, "You can't hide behind your jokes forever, Gabriele."
There is a brief scene in which Fr. Amorth walks away from the Colosseum in Rome. In an interview, Russell Crowe confirmed that this was a "wink and a nod" to Gladiator (2000).