Michael Fassbender interview in SuperHeroHype

"I haven't based [Magneto] on [Ian McKellen] at all actually, and I hope the fans aren't going to be disappointed by that, because I know he's rated as #1 comic villain, well he's up there anyway, I just tried to steep myself in the source material, the comic books, and there's so much. He's such a fascinating guy, and how he becomes Magneto is a really interesting journey. The fact that he has been let down so many times by human beings and this distrust that he has. You can see where it's coming from. Obviously, what was happening in the concentration camps and then he escaped from there with Magda, fell in love with her, then their child got burned and he ended up killing the whole town and she's like 'Whoa, this is too heavy for me, I gotta leave.' So the love of his life left him."

"We see the stuff that's happening in the concentration camp, you see that, but you don't see that relationship [with Magda]. I've just held that for my own personal use [in developing] the character, so when we meet him at the beginning of this film, he's definitely a lone wolf. He's on a mission and he's uncompromising and then he bumps into Charles, literally collides into him, and then it's the development of their relationship and really exploring that during the '60s and the Civil Rights Movement, Martin Luther King an Malcolm X."

"That was a big draw when I heard that [James McAvoy] was going to be playing Charles Xavier. I thought, 'That's really interesting," being a big fan of his and we briefly worked together ten years ago on 'Band of Brothers.' I like him a lot. He's really easy to work with, he's just a normal practical worker, and we sat down and made sure that the relationship had all the elements that we wanted and that the progression was a good one, that there's no filler scenes and every scene is there for a reason. We did a lot of writing together with Matthew, each day we'd sit down and make sure the scene was tight and that it was dealing with everything we wanted to deal with, the fact that they're unsure of each other, sniffing each other, almost like dogs, and then there's a real respect and liking for each other, and then the relationship fractures."

http://www.superherohype.com/news/articles/126195-exclusive-michael-fassbender-talks-about-magneto