When you're given the assignment to write, for example, 'Spider-Man,' the concept, characters and environment are all laid out for you. Everything is pre-established, and your sole responsibility as a creator is to craft an exciting, entertaining, hopefully original adventure, to add layers and colors to a canon that already exists.
In terms of the 'X-Men: Days of Future Past' movie, Bryan Singer has been a part of the X-Men family from the first movie. He knows about the comics canon and how it relates to his work as a filmmaker. He's more well versed in the canon than most, as are the people that are working with him.
When you're an adolescent, you suddenly wake up one morning and your body is an enemy. There are hormonal changes, physical changes, emotional changes. People are saying to you, 'Now you have to make the decisions that define the rest of your life.' The X-Men takes those elements and pushes them one giant step farther.
Even when I was growing up as a young boy, when I was playing schoolboy football, there were other guys who were as good as I was, maybe some even better technically. But I was prepared to stick to what was going to make me become a professional football player when I left school, and that was a lot of sacrifice and because my attitude was right.