“Band of Brothers” was a massive undertaking, featuring a cast of at least 23 actors in main roles, with dozens more supporting. Simon Pegg, Jimmy Fallon, Dominic Cooper, Tom Hardy, and Jamie Bamber also joined Fassbender and McAvoy in the ensemble. It seems that the miniseries, all about the camaraderie between soldiers, created a similar attachment between the members of its massive cast. 

Just prior to the release of “Dark Phoenix,” the tenth X-Men film, Fassbender pointed out that the two actors had indeed met during “Band of Brothers,” some 20 years ago. McAvoy also shared that, “there was chemistry there from the beginning, and that is why we agreed to do this.” McAvoy, it seems, was cast first, while Fassbender came in after to audition with him. Everything clicked immediately. Fassbender said: “James was at my audition when I first auditioned for Erik [Lehnsherr, a.k.a. Magneto] as well. James was kind enough to be there for me for that.”

Their audition involved a lot of improv, and McAvoy and Fassbender immediately developed an actorly rapport, allowing them to merely have a conversation. The heady, extensive X-Men mythos wasn’t important. What was important was that the two characters felt like friends. McAvoy said: 

“All the scenes were so natural. […] There were scenes when we were shooting that were almost two guys shooting the s*** about who they love, the people that they were in love with, and it did feel very comfortable. It just felt like a couple of guys and I think that was why it ended up working.”

By the time the X-Men series had progressed to “Dark Phoenix,” however, the two characters had a much more fraught relationship.