Filming
Principal photography began on February 5, 1996, with the script "still in flux"; according to The New York Times, "ego clashes, budget overruns and long delays plagued the project." Pitt "threatened to quit early in the shoot, complaining that the script was incomplete and incoherent" and later "denounced the movie as 'the most irresponsible bit of film making – if you can even call it that – that I've ever seen.'"
To ease Pitt's frustrations, the producers hired screenwriter Terry George one week before principal photography was to start. George's primary purpose was to concentrate on helping further develop Pitt's character Frankie McGuire, and what the actor thought was a superficial outlook on the IRA situation.
In March 1996, Pakula hired screenwriter Robert Mark Kamen to provide rewrites during production. Kamen noted the difficulties on set: "They were running out of script to shoot. They had a script that wasn't acceptable to either actor. Alan [Pakula] didn't start with a script that everyone had signed off on, we were flying blind....It was scary".
Kamen met with both Ford and Pitt separately to discuss their ideas about improving the script, then he and Pakula would review the material, Pakula would add his own critical comments, and Kamen would go off to write the new scenes.
Contrary to the rumors that were reported at the time, Kamen insisted that both actors were agreeable to each other. "It wasn't the tension between them that made things tense. It was the tension each had with their own parts."
In June 2023, Ford admitted that he was partially at fault for the tension between him and Pitt, stating: "I understand why he wanted to stay with his point of view, and I wanted to stay with my point of view, or I was imposing my point of view, and it's fair to say that that's what Brad felt, it was complicated".
According to Pakula, one problem was that the film's plot did not fall along conventionally simple Hollywood lines, as Ford and Pitt were both playing "good guys" according to each of their own distinct moral codes. The New York Times characterized Ford's character as "the upright American cop who deplores violence" and Pitt's as "an I.R.A. gunman for whom violence is a reasonable solution to his people's 300 years of troubles." Pakula compared his intent with the two characters to that depicted in Red River, a 1948 western in which John Wayne's character Thomas Dunston is defied by his young protégé Matt Garth, played by Montgomery Clift.
The Devil's Own was filmed on location in New York City, and in Newark, Hoboken, Jersey City, Bayonne, Sandy Hook and Montclair, New Jersey and Greenport, New York on Long Island. Among the metropolitan location highlights were The Cloisters Museum at Fort Tryon Park, Brookdale campus of Hunter College, Isham Park, St. Barnabas Episcopal Church in Irvington, the Staten Island Ferry, Wollman Rink, The Old Town Bar, the shipyard in Red Hook, Piper's Kilt bar in The Bronx, the 1889 Bar & Grill, and the Waverly Coffee Shop, with sound stages utilized at Kaufman Astoria Studios in Queens, and in Hell's Kitchen, Park Slope, and Chelsea Piers, For security reasons, the Northern Ireland scenes were instead shot in the Republic of Ireland. The opening scenes were filmed at Port Oriel, Clogherhead, County Louth, Republic of Ireland. The Belfast shootout scenes were filmed in Inchicore, Dublin in July 1996. Other location shoots in Ireland were in the Wicklow Mountains.
Two months before it opened, the film was still unfinished: Pakula, unhappy with the final scene ("a showdown on a boat with a cargo of Stinger missiles"), called the actors and crew back for additional filming. The scene was "rewritten and reshot over two days at a studio in California."