According to Channing Tatum, he and Mark Ruffalo spent an intensive five to six months training for wrestling, which took its toll on the actors. During one particularly physical take, Channing insisted to Ruffalo "to just slap the shit out of me and get it over with", which resulted in Channing's eardrum accidentally getting popped. The take is in the film.
Before filming a particularly dark scene, Bennett Miller made Steve Carell write on a piece of paper the thing that he hates the most about himself and then put it in his pocket. Miller told Carell, "Just have it right there, and know that it's in a place where, if I was a dick, I could just grab it." According to Miller, the result is the favorite thing that he has put on film.
Mark Ruffalo said the cast and crew were so afraid of Steve Carell as du Pont, that they tried to avoid interacting with him. Ruffalo said, "Everyone sort of stayed away from him. John du Pont was repellent, so the way they designed Steve's look, it made him repellent, and you didn't know what to say to him, or how to act around him. You never felt comfortable."
In the 1984 Olympic games, Dave Schultz and Mark Schultz were accused of "excessive brutality" in their Olympic matches, and a special official was assigned to monitor the rest of their bouts. Two of their opponents went straight to the hospital after their matches. Mark, 23, broke the left elbow of the 180.5-pound European champion Baris Karabacak thirty seconds into the match. Dave sent Yugoslavia's Saban Sejdi to the hospital with a knee injury incurred during their 163-pound match.
Steve Carell studied video footage of John du Pont for hours. He told reporters after the film's screening at the Cannes Film Festival that "I watched as much as I could, I read as much as I could about him, and tried to get semblance about the type of person he was." According to Bennett Miller, Carell's career as a comic actor hadn't suggested that he was right for the role, until he had lunch with the actor. Miller said upon Carell's casting, "I think all comedians are dark."
Channing Tatum and Mark Ruffalo would film all day, and then go to intense wrestling practice. By the end of the shoot, they were so exhausted, that according to Tatum, he and Ruffalo cried after their final practice.
Because the project took so many years to get off the ground, many actors were considered for the lead roles. Heath Ledger, Ryan Gosling, and Bill Nighy were strongly considered for the lead roles in the early stages of production.
When the wrestlers are shown watching an Ultimate Fighting Championship, the year is 1987, although the match shown is UFC 8, which took place in 1994. The winning fighter in the match shown is Gary Goodridge, who Mark Schultz would later fight when he joined the UFC, and competed in UFC 9 in Detroit, Michigan.
The writers of the movie, Dan Futterman and E. Max Frye, didn't actually work together on the screenplay. Back in 2006 and 2007, Bennett Miller approached Frye and started working on a draft, but a writers' strike happened, and the project was paralyzed. Miller wanted to keep working on it, and contacted his childhood friend and writer of Capote (2005), Futterman, who finished up Frye's work.
Bennett Miller confirmed in an interview, that he got the idea to do the film after he was given newspaper clippings over the events that took place at Foxcatcher Farm in 1996 from a fan at a DVD signing of his 1998 documentary The Cruise (1998).
According to a documentary about the making of the film, all the three leads were Bennett Miller's first choice for the main roles. However, he was a little apprehensive to contact Channing Tatum and Mark Ruffalo, since Tatum had passed on the film a few years before the movie went into production, feeling he wasn't suitable for the role. In Ruffalo's case, he turned down the Perry Smith role in Capote (2005), also directed by Miller, three times.
According to the documentary Team Foxcatcher, John DuPont became increasingly paranoid in the weeks leading up to the murder of Dave Schultz. He thought Schultz was in his walls and using tunnels under his home to "mess with him". Among other things, Mr. DuPont came to hate the color back saying it was the color of death. He got rid of all his black horses, people living at his home who drove black cars had to get rid of them and he fired all the African American wrestlers from the team.
In the movie, you see that Dave Schultz has PU Kids written on his hand. This was to remind him to pick up his children. However, he had it written on his forehead as well. This was observed the morning of the day he was murdered.
Mark Schultz said that after John du Pont went to prison, du Pont sent a private investigator over to Mark's house. The private eye was to ask Schultz for help in getting out of prison. Schultz filmed it, then promptly slammed the door, and sent the footage to the prosecutor's office.
In real-life, John du Pont murdered Dave Schultz eight years after Mark Schultz lost the Seoul Olympics. After the murder, du Pont retreated to his mansion, where he negotiated with police over the phone for two days, before being arrested after his house's power was cut, and he stepped outside.
According to Steve Carell, the real John du Pont was known for even more outlandish behavior than what is shown in the film, but he and Bennett Miller wanted his madness to be gradually revealed to the audience.
Bennett Miller on casting the lead: "I didn't want to hire an actor who you might presuppose was capable of murder, because one thing that was repeated among many, many people who were there, was that nobody believed du Pont was capable of doing what he did."
One of the young wrestlers that Dave Schultz trained was Kurt Angle, who would end up being a WWE and TNA wrestling star. He was part of Team Foxcatcher at the time of the movie's events, preparing for the 1996 Summer Olympics. He would go on to win gold for Team U.S.A. in the heavyweight class, and dedicated the medal to Dave Schultz's memory.
After John du Pont died in 2010, according to his will, he was buried wearing his Foxcatcher singlet in a coffin that contained his wrestling medals from the events, like the one depicted in the movie.
Sienna Miller claimed that as filmed, Nancy Schultz's 911 call after Dave Schultz's shooting by John du Pont, was a major scene based on the actual recording of the call, though it is presented silently in the completed film. She further stated that filming the murder scene was extremely pressing, since she was recovering from recent childbirth when she filmed the scene, and the real Nancy Schultz was on-set watching her performance.
Mark Schultz's version of the events, on which the movie is based, written with David Thomas, was also published in 2014, titled "Foxcatcher: The True Story of My Brother's Murder, John du Pont's Madness, and the Quest for Olympic Gold."
The real Mark Schultz asked to have the gay molestation implications cut, because he and du Pont never had that kind of relationship, and he found the inaccuracies insulting. But the director decided to keep them in, explaining that they were meant "to give the audience the feeling that du Pont was encroaching on [Mark's] privacy and personal space." When the movie came out, Schultz himself took to Twitter and Facebook to denounce it. He has since apologized for the outburst, but maintains that the film is grossly inaccurate.