Patrick Stewart and Ian McKellen were performing in a touring production of "Waiting for Godot" when Bryan Singer approached the actors about reprising their respective roles as Professor X and Magneto. According to McKellen, both men were utterly shocked as they thought they'd passed their roles on to James McAvoy and Michael Fassbender, and would never play the characters again. Both Stewart and McKellen were delighted to return to two of their most popular roles, and to work with the younger actors playing the same characters as well.
Bryan Singer filmed the mutant Quicksilver's scenes in a special format of 3600 frames per second. This was done to showcase Quicksilver's speed ability: 3600 fps is 150 times slower than normal film (which is at 24 fps), so Quicksilver will be seen moving to 150 times as fast as normal.
The addition of Evan Peters as Pietro Maximoff/Quicksilver to the cast sparked wide discussion over the direction of the character who is also slated to appear in Avengers: Age of Ultron (2015). Quicksilver had been discussed previously as a potential character in both X-Men: The Last Stand (2006) and The Avengers (2012), but legal complexities over the license to the character resulted in his omission from both films. However, in May 2013 both Marvel and Fox Studios announced a resolution to the previous legal issues, and that Quicksilver would appear in this film as well as an Avengers sequel, though under certain parameters: no reference to Quicksilver's membership in the Avengers can be made in an "X-Men" film, and no allusion to his relations to the X-Men or Magneto (the character's father) can be made in an "Avengers" film; the rights agreement between Fox and Marvel even goes so far as to stipulate the character cannot be referred to as a "mutant" in any Marvel film. Additionally, the day after the announcement of Peters's casting, Marvel and Fox entered into a legal standoff over provisions of the rights agreement for the character, including the issue of whether Peters would be allowed to portray Quicksilver in any other film outside the "X-Men" franchise, possibly necessitating a second actor to play Quicksilver in any Marvel film, resulting in two different versions of the same character appearing in two competing film series. Ultimately, Fox and Marvel decided to cast different actors in the part for the "X-Men" and "Avengers" films, with Aaron Taylor-Johnson taking on the role in the latter sequel, thus preventing any connection between the two franchises and keeping the X-Men confined to a separate universe from those of the Marvel cinematic universe.
When Matthew Vaughn was going to direct, he was going to make the film a direct sequel to X-Men: First Class (2011) and have it set in the 1970s. Early ideas included an opening with the Kennedy assassination being caused by Magneto, and mutant encounters set in the Civil rights movement/the Vietnam War. When Singer took over, he integrated these concepts into a viral marketing campaign to set up the action of the film. In this alternate history, Magento is arrested and imprisoned for the assassination of Kennedy, but maintains his innocence. The "Bent Bullet" Theory (a reference to the real life "Magic Bullet" Theory criticized by conspiracy theorists) holds that the Warren Commission determined that Magento manipulated Lee Harvey Oswald's bullets to kill the President in retribution for the murder of the mutants Azazel and Tempest (played by Jason Flemyng and Álex González in X-Men: First Class (2011) respectively) by the CIA. Conspiracy theorists, based on Magneto's testimony, insist however that Magneto had tried to prevent the murder of Kennedy, and that the true shooter was not Oswald, but Mystique in disguise who, with the help of Emma Frost (played by January Jones in "First Class") framed Magneto, and manipulated Jack Ruby into later murdering Oswald. The theory also posits that Mystique offered to double as Kennedy in an attempt to grab power, all of which backfired horribly, leading to anti-mutant hostilities.
Including his cameo in X-Men: First Class (2011), this will be Hugh Jackman's seventh portrayal of Logan/Wolverine, raising his own record for the most times a comic book character has been played by the same actor in theatrical films. He will also be the only actor to appear in the entire X-Men film series.
The filmmakers selected the "Days of Future Past" storyline because it would allow the filmmakers to reconcile any continuity dissonances within the "X-Men" film series. The time-travel element also allowed actors from the original film series and the intended reboot that was X-Men: First Class (2011) to appear in the same film together.
The release of the teaser trailer for this film ignited such interest, director Bryan Singer made the unprecedented move of recording an actual commentary track to it the following day, explaining the significance of certain scenes and offering more insight into what to expect from the film.
Anna Paquin filmed scenes as Rogue and, though heavily featured in the trailers and advertising, her scenes were cut from the final film for pacing reasons. Bryan Singer later announced they she may still appear in the film.
According to Bryan Singer, he had a two-hour discussion with James Cameron, director of the time-travel films The Terminator (1984) and Terminator 2: Judgment Day (1991), about how to make the time-travel concept feasible and workable within the film. The concepts the two discussed included alternate universes and string theory (a field of quantum physics that define multiple universes).
Bryan Singer and Peter Dinklage describe Boliver Trask as a peace-lover: "He feels that humanity will go on fighting each other, unless they can find a common element to unite against; he sees the advent of the mutants as a way to unite people. He sees what he's doing as a good thing - his ambition is definitely blind. He's strove all his life for a certain respect and attention."
Jason Flemyng was originally set to reprise his role as Azazel when Matthew Vaughn was still set to direct. When Vaughn left, the storyline was dropped in favor of the time travel/crossover storyline, and Azazel's role cut from the script to accommodate characters from the original "X-Men" film series.
According to Peter Dinklage, Bryan Singer picked him to play Boliver Trask because of his height: "With my dwarfism, I'm a bit of a mutant. I can't move metal or anything, but I thought of it as self-loathing. Deep down, Trask is quite sensitive about that aspect of himself."
Production designer John Myhre described the future Sentinels as having evolved from machines into biomechanical weapons: "They are almost made up of magnetic plates slapped over one another, imagining that the plates could contract or grow, so the Sentinel can be skinny to get through a small space or the plates can open up to become a bigger shape. They have become virtually unstoppable - the ultimate version that can actually, in principle, stop the X-Men."
Bryan Singer based Boliver Trask on Adolf Hitler: "As Hitler used the Jews as a scapegoat to bond the darker parts of Europe, he's doing the same thing with mutants. But he wasn't a six foot, perfect blond Aryan - he was a short, funny looking fellow!"
During Magneto's fight with the presidential guards, there is a painting behind him. This painting is "Liberty Leading the People", a French painting made to commemorate the 1830 French Revolution, and a symbol of rebellion.
Kiefer Sutherland was rumored to play the role of Nathaniel Essex. The scene features Essex speaking to Logan in boxing gym. The scene took two days to shoot and is rumored to be the after-credits scene.
The trivia items below may give away important plot points.
Bishop, like in the comics, is an actual time-traveler and aware of the changes to the timeline. This refers to the "Age of Apocalypse" comic, where he was the only one who could stop Apocalypse from changing history. This also sets up Apocalypse as an antagonist for X-Men: Apocalypse (2016).
In the "Days of Future Past" comic it was Shadowcat who went back in time; in the film it's Wolverine. According to writer Simon Kinberg, Kitty was intended to be the time-traveller but it didn't work out: "Kitty in the era of young Magneto and Xavier, would have been negative 20 years old. The reflex response to that was a character who doesn't age. Wolverine is the only character who would look the same in 1973 as he does in the future." Thus, Wolverine was picked for being an ageless immortal character who would bridge past and future.
Bryan Singer based the time travel in the film on string theory: "Until an object is observed, it hasn't really happened yet. The time-traveller whose consciousness travels through time I call The Observer, and until the Observer returns to where he travelled from, the result hasn't occurred yet. So he can muck about in the past and it isn't until he snaps back that the new future is set. As a result, we have parallel action, and there's underlying tension because there's always that threat Wolverine's consciousness could return to the future and leave the world in an even darker place."
In the "Days of Future Past" comic Mystique tries to assassinate Senator Robert Kelly leading to the genesis of the Sentinel program. In this film Mystique remains the assassin, but as Kelly had appeared in X-Men (2000) her target is now President Richard Nixon.
The bullet wounds that appear on Wolverine's chest on his arrival to the 1970s is in the form of the Big Dipper. This is a homage to Hokuto no Ken (1984), whose protagonist Kenshiro has the same scar pattern on his chest.
According to Simon Kinberg, this film unites ALL the future mutants: "It's this dysfunctional family and these desperate people who are outcasts in their own lives, and they come together, and that's not as emotionally satisfying as an outcast on their own."