(at around 18 mins) The script called for Logan to wake up in 1973 in boxer shorts. Hugh Jackman vetoed this, in favor of waking nude, saying, "In Australia, if you're next to a really good-looking girl, you're not getting out with boxer shorts on, or briefs, or anything!"
Sir Patrick Stewart and Sir 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 had passed their roles on to James McAvoy and Michael Fassbender, and would never play the characters again. 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.
According to Peter Dinklage, Bryan Singer picked him to play Bolivar Trask because of his height, stating, "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."
Including his cameo in X-Men: First Class (2011), this is Hugh Jackman's seventh portrayal of Logan, a.k.a. Wolverine, which raised his own record for the most times a comic book character has been played by the same actor in theatrical movies. Jackman is also the only actor to appear in every X-Men movie in the franchise up to Logan (2017), which he announced as his last time playing the character. However, he made an exception for Deadpool & Wolverine (2024).
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 franchise. The time-travel element also allowed actors and actresses from the first three movies, and X-Men: First Class (2011), to appear in the same movie together.
Len Wein: (at around 20 mins) Creator of the Wolverine character plays one of the Congressmen at Trask's hearing. So does Chris Claremont.
Newton Thomas Sigel: (at around 1h 5 mins) A disguise Mystique takes to escape from the Paris summit. Sigel is the cinematographer for all of the Bryan Singer-directed X-Men movies.
Zoë Kravitz: (at around 37 mins) Appears as her character from X-Men: First Class (2011), Angel Salvadore, in an autopsy photograph in Trask's files, viewed by Mystique.
Chris Claremont: (at around 20 mins) One of the Representatives at Trask's hearing. Claremont was the writer of the original "Days of Future Past" comic, and was brought on as a consultant.
Bryan Singer: (at around 1h 2 mins) When Mystique jumps out of the building and onto the ground below after attempting to kill Trask, Singer is a photographer, who is briefly seen holding up a camera.