McKellen and Patrick Stewart will reprise their roles as Magneto and Professor X in Bryan Singer's superhero sequel X-Men: Days of Future Past. The pair will play the future incarnations of Fassbender and James McAvoy's X-Men: First Class mutants.
McKellen told The Huffington Post: "What a gas! I mean, Patrick Stewart and I have just been playing Waiting for Godot throughout England and in the West End of London. So, we've stayed in touch. We can't believe it.
"I thought these were Fassbender and McAvoy's parts, but - no, no - we're back. We'll give them a run for their money. I mean, I haven't read the script, so I don't know how much we've got to do. But, to be back in that world, and for Bryan to be doing it - with all the success he's had in the meantime - it's wonderful. Wonderful."
"I thought he was better than me. But, we weren't quite playing the same character because he was so much younger than me," McKellen said. "And the scope of Magneto, one day a Jewish actor will play it. Clearly, he's Jewish.
"We first meet him in at Auschwitz and there's scope for all sorts of people to pay Magneto. But, I was just the lucky guy who got to play him first."
X-Men: Days of Future Past, which will also star Jennifer Lawrence and Nicholas Hoult, will open in cinemas on July 18, 2014.