Directed by Bryan Singer, the film follows Hugh Jackman's superhero Wolverine as his consciousness travels back to his 1970s-era body and tries to persuade benevolent professor Charles Xavier, and Xavier's rival Magneto, played by Michael Fassbender, to team up and stop a war profiteer from using machines he developed to destroy all mutants.
McAvoy and Fassbender previously played the roles of Charles and Magneto in the 2011, 1960s-set prequel, X-Men: First Class, while Jackman, Stewart and Ian McKellen played Wolverine, Charles and Magneto in 2000's X-Men, 2003's X-2: X-Men United and 2006's X-Men: The Last Stand.
Days of Future Past features McAvoy, Fassbender, McKellen and Stewart as the characters in their respective time periods, with Wolverine acting as a bridge between the eras. One memorable scene shows the two incarnations of Charles briefly meeting face-to-face.
"I'm not quite sure how it came about that we were nose-to-nose like that, but I can't now think of any other possible way of making the scene work because you are looking into the eyes of yourself," Stewart said at a recent New York press conference.
"It was James' first day of work on the movie and it was my last day of work on the movie. My bags were packed and I was ready to get out of Dodge. I don't recall rehearsing it. We knew the lines and they rolled the camera. I think it was 40 minutes work, as far as I can recall. I shouldn't have said all that. I should have said we worked on it for weeks. The two of us and Bryan and the crew. But it actually wasn't like that at all."
McAvoy, who was sitting beside Stewart on the panel, confessed, "Getting to come and do my version of a character that he has been in charge of for 14 years -- at his face -- was quite nerve-wracking.
"So, you've got the two choices -- either you get nervous and let that overcome you or you get quite excited about the fact that you might fail, so it was good fun," McAvoy said.
Turning to Stewart, he revealed, "I've been watching you for years -- since I was 10 or 11. ...
"And your characteristics and idiosyncrasies, I know them quite well, as a performer, anyway. But the key thing was watching the empathy that pours out of you in the previous movies and I hoped in the First Class to be able to emulate that particularly because it is kind of the prime characteristic of Professor Xavier -- this willingness to care and reach out and help," McAvoy said.
"It made me feel that I'd really like to get back and shoot all the other movies again, now that I know exactly where I came from and what I was," Stewart added. "I could get so much more James McAvoy into that performance."
Co-starring Jennifer Lawrence, Nicholas Hoult and Peter Dinklage, X-Men: Days of Future Past was No. 1 at the North American box office when it opened this past weekend.