While the Marvel comic character of Quicksilver will appear in both "X-Men: Days of Future Past" this year and "Avengers: Age of Ultron" next year, the parts are played by different actors and are said to be totally different takes on the role.
At one time though, very early on in the story breaking process and before the script had begun to be written, another character was lined up for the role in 'X-Men' that Quicksilver now serves in.
That character was a young Juggernaut, the mutant who is physically unstoppable once in motion. Asked about the character by Empire, director Bryan Singer says:
"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... [writer Simon] Kinberg had originally employed a young Juggernaut in the same function."
Evan Peters plays the Quicksilver role in 'X-Men' and details of his scene have been revealed - the character helps Charles Xavier and Wolverine break Magneto free from the Pentagon in 1973. Vinnie Jones played Juggernaut in "X-Men: The Last Stand," but the character will not be back in 'Days' and the interview doesn't explain why they went with Quicksilver.
Explaining the Quicksilver character, Singer also hints that the relationship of Quicksilver and Scarlet Witch to Magneto in the comics, namely that the latter is their father, may only be suggested and not explicitly stated in the film:
"Everybody's so slow to him, it gives him an attitude. He's a little impatient. When [Magneto and Quicksilver] first meet, there's a cool, 'Do I know you?' moment."