Paul Douglas comments that St. Peter's is bigger than Yankee Stadium. In fact, the Yankee Stadium of 1952 could fit into just the central aisle of St. Peter's.
This film was a flop at the box office for MGM, resulting in a loss of $918,000 ($8.4M in 2017) according to studio records. While recent Angels in the Outfield (1951) (also directed by Clarence Brown and co-written by Dorothy Kingsley) was a box office success, When in Rome (1952) lost money at the box office. Of course, movies like When in Rome were never big hits, not enough to justify a trend at least, but they got made, here and there, in the heyday of Hollywood, by both Hollywood liberals and conservatives who hadn't yet found a way to politicize faith. One of its key writers was Dorothy Kingsley, who was herself a devout Catholic and took Robert Buckner's story to heart. She would rarely get sole screenwriting credit since she was usually brought in to doctor scripts started by others. It's clear that this was one such job she was more than happy to do. When in Rome may harken back to a different era of gentler films but it still packs a punch with its tale of redemption.
The transatlantic ship depicted in the film taking Joe and Father John to Genoa is the S.S. Ionia - per the name seen on the lifeboat. It was built in 1912 in the UK and named the S.S. Digby. After four subsequent owners it was acquired by the Hellenic Mediterranian Lines of Greece in 1947. Sold to a Panamanian line in 1965 and renamed the S.S. Ionian, it capsized and sank that same year off Jakarta, Indonesia.
Much of When in Rome (1952) was shot on location in Rome. Roman locations included The Spanish Steps, the Colosseum and the churches of Santa Maria Maggiore, St. John Lateran and St. Paul Outside the Walls. Some interior shots of St. Peter's Basilica were also made.
Two years after When in Rome (1952), Van Johnson was dropped by MGM as his career hadn't taken off like studio execs hoped it would. He would give his best performance shortly thereafter in The Caine Mutiny (1954), prompting many to consider his acting talents had been underused.