According to Franco Zeffirelli's autobiography, The Beatles were asked to appear in this movie in the main roles, but were unable due to scheduling conflicts. Zeffirelli also screentested Al Pacino for the role of Francesco, but rejected him due to his theatrical overacting style.
The scene in which St. Francis renounces his birthright was filmed in the forecourt of the Church of San Rufino in a portion of Assisi across town from the Basilica of St. Francis. The church of the Porciuncula was a set built in a meadow. The interior of the Pope's church was built on a soundstage in the studio, but the massive depiction of Christ that was behind and above the Pope's throne is an actual mosaic from a church in Sicily which was matted into the shot.
The dye stains on his clothes (red and blue) are linked to the city colors of Assisi. The flags on the soldiers show a red/blue vertical split.