James McAvoy and Bryan Singer talked candidly about their upcoming film "X-Men: Days of Future Past" in an interview with Collider.

The actor talked about Professor X's journey from the first "X-Men: First Class", explaining how the character has grown from the first film.

"You'll find him very different. Not just because he's got long hair, but because of what he is and how shaky his soul is. That's really the key to him in this one, that his power, which has always been seen as his psychic ability or great intelligence or whatever, really what I think it is empathy. That's his greatest power," McAvoy told Collider.

"But he's lost the ability to be able to empathize with other people, because it's too painful for him," the actor added. "Why? Because he's been given all of his own pain to deal with, by Erik and by, as we said, Raven as well. It's not just the loss of Erik. It has a lot to do with Erik, a lot to do with his relationship with Erik and their love for each other, but it's also equally as much to do with his love for Raven, and the fact that he was sort of abandoned at the end of the last movie by both of them. Not just abandoned, but horribly injured."

While McAvoy talked about the evolution of his character, Singer broke down how he made the time travel in the sure cohesive and easy to follow.

"I wanted it to make sense, and very often with time travel, and this is a rare film where past and future are co-existing, so I had to create a set of rules where that made sense to me," Singer explained to Collider. "So that's what I set about I do I created a very specific set of rules. When do things change? Who observes the change? Who has no memory of the change? Who has no memory of what was and who does, and how that works."

"I think I cracked it," he added. "I think I figured it out but that was my first, when Matthew left the picture and I took it on, that didn't exist, there was a structure but there was no concept of time travel or how it works. So until I figured that out I had some misgivings about doing the movie. Once I figured that out I was very hooked, I felt like they had me cause I was like now I know how to do this and shoot this."

"X-Men: Days of Future Past" is slated to hit theaters on May 23.

You can check out McAvoy's full interview with Collider here or Singer's here.