Star Wars has had decades to cement itself as one of the most popular and powerful movie franchises, meaning that it's easier than ever for it to recruit top-level talent. Some notable stars who appeared in the prequel trilogy and more recent movies include Liam Neeson, Christopher Lee, Andy Serkis, Lupita Nyong'o, Mads Mikkelsen and Forest Whitaker, to name a few. Conversely, there have been plenty of instances where castings haven't worked out. For instance, Michael Fassbender was approached to appear in Star Wars: The Force Awakens, but it never progressed past talking due to scheduling conflicts. He explained:
We talked about a role. We had a conversation. I'm pretty sure I was busy doing something else in the summer he was kickstarting that.
Michael Fassbender made this revelation while appearing on MTV's Happy Sad Confused podcast, citing that he was already working on something else when "he" (presumably director J.J. Abrams) approached him for The Force Awakens. Of course, even if the actor hadn't been tied up with another project doesn't necessarily mean a deal would have been finalized...though it's hard to imagine anyone turning down Star Wars if given the perfect opportunity. Since Episode VIII has already shot and the Han Solo spinoff has most of its main cast assembled, maybe there will be an opening in Episode IX for him. Fun fact, it was also rumored over a year ago that Fassbender was being considered to play Boba Fett.
Although Michael Fassbender didn't provide any details about the character he was supposed to play in The Force Awakens, several options come to mind. He's only two years older than Oscar Isaac, so perhaps he was considered to play ace pilot Poe Dameron. However, given his past experience with villainy in the X-Men movies and 12 Years a Slave, Fassbender would have also been a good fit for either Kylo Ren or General Hux, two of the First Order's top leaders. Those roles eventually went to Adam Driver and Domhnall Gleeson, respectively. If I had to choose, Fassbender would have been a great fit for Hux. Not a dark side user, but still a calculating villain serving Supreme Leader Snoke. Whoever Fassbender was considered for, you can bet it wasn't to play anyone minor. Although, now that I think about it, maybe he could follow in Daniel Craig and Tom Hardy's footsteps by cameoing as a stormtrooper for fun in the future.
You can currently see Michael Fassbender as Callum Lynch/Aguilar in Assassin's Creed, and he'll appear next May in Alien: Covenant as the androids David and Walter. As for the Star Wars franchise, Rogue One is also currently playing in theaters, and Episode VIII will be released on December 15, 2017.