According to Mstars News on Feb. 3, in an interview with Bryan Singer about "X-Men: Days of Future Past", the director was asked if had replaced Juggernaut with the Quicksilver character, as portrayed by Evan Peters, in response to Joss Whedon's "The Avengers: The Age of Ultron" planned casting of the same character. Singer stated that he had already been scripted into the film and Marvel was the entity that informed him of that awhile ago.
"Quicksilver was always in our script. Marvel were informed early on that I was using him. The way it'll play out in our movie versus what I believe they are going to do is so different. And this is not an enormous role - he's only on one beat of the movie".
The reference to the "one beat" is in regards to a scene in the film where Quicksilver, Charles Xavier, and Wolverine are in the kitchen of the Pentagon. Their objective would be to break the young Magneto, played by Michael Fassbender, out of the facility. There is a guard entering the elevator to bring food to Magneto when all of a sudden sees Quicksilver appear with a smirk. Some may be reminded of another forgotten mutant in a previous "X-Men" film where he teleports his way into the President's Oval Office. It seems that in both past and present the X-Men seem to have experience breaking into iconic government facilities.
Screenwriter Simon Kinberg had originally brought upon Juggernaut to partake in the same jailbreak scene, however it is not known at what point in the writing process that Juggernaut was replaced by Quicksilver.
Also, Peters brings attention to the encounter his character has with Magneto where for a moment they have expressions on their faces that seem to indicate they may know one another. This could reference their blood relation to each other in the comic book world that Joss Whedon brings about in "The Avengers: Age of Ultron" but this would not be possible with Marvel Studios because Fox own's the rights to the "X-Men" franchise.
Two Quicksilvers in two different franchises, but never the twain shall meet.
"X-Men: Days of Future Past" releases in theaters on May 23.