California Doubling: Both films are set in Boston, but, apart from the flyover establishing shots, were filmed in Toronto. The CN Tower and Toronto Dominion bank locations can be seen in various shots.
In a more specific example, the church scene at the start of the first film was shot in an Anglican Church rather than a Catholic Church, due to the producers being unable to convince any Catholic diocese to allow them to film on their property. The Anglican traditions are considered about as close to Catholicism as non-Catholic Christian churches get, but you can still spot a few differences in the service as shown.
In Spanish, the movie is known as Los santos del infierno, or "The Saints of Hell".
In German, it's "Der Blutige Pfad Gottes", meaning "The Bloody Path of God".
In Japanese, it's Jyokeinin, or "Executioners".
The Danza: Carmine DiStephano plays Augustus DiStephano, a retired mob boss. Also, David Della Rocco is played by none other than a man named David Della Rocco.
Dawson Casting: A rather bizarre example in that, in the sequel, Judd Nelson plays the son of the Big Bad of the first film, played by Carlo Rota, when Nelson is actually a year and a half older than Rota (Nelson was born on 1959 and Rota was born on 1961).
Development Hell: The second film took ten years to make due to a lack of funding and Troy Duffy's ego. The third film, thus far, seems to be in the same position. Duffy has said if negotiations go through, the story may be continued in a TV series, but the progress seems to be going slowly.
Enforced Method Acting: Apparently, Billy Connolly was so stoked to be Playing Against Type so radically in the role of Il Duce and having so much fun playing a psycho with guns strapped all over him that they had to add the huge cigar the character constantly smokes because unless they stuck something in his mouth he simply couldn't stop smiling.
Fake Russian: Fat Man (Yuri Petrova) is the only Russian played by an actual Russian.
Invisible Advertising: The film was all ready to be released, but unfortunate timing meant it had to be released quietly. The second movie was set up this way at first, but positive reception among the fans gave it two wider releases.
Troy Duffy expressed interest in casting Stephen Dorff, Brendan Fraser, Nicky Katt, and Ewan McGregor, with two of them as the brothers, but no decisions were finalized. Duffy flew to New York to meet with McGregor and offer him one of the lead roles. At a night out, Duffy got drunk and they got into an argument over the death penalty and McGregor passed on the film. This led to Miramax putting the film in turnaround.
Troy Duffy dismissed several casting choices for the film — he refused to meet with Brad Pitt because he'd already played an Irishman in The Devil's Own and rejected Keanu Reeves and Ethan Hawke, calling the former "a fucking punk" and the latter "a talentless fool".
Probably the most known was that the movie was supposed to be released by The Weinstein Company, as Harvey took interest in the script (as rumors state, anyway; as some say he didn't even read the script, just that it was hot property) and even offered extensive goods to Duffy such as buying a bar Duffy worked at, to even allowing Duffy's band The Brood to have a record deal to do the film's soundtrack. Unfortunately, due to Duffy's infamous ego, all of these plans collapsed and the film was instead given to another third-party company.