The 37-year-old star opened up about his experience while filming the psychological horror thriller, reported Contactmusic.
"I got the role quite late in the day, so I had to do nine times the amount of preparation in really less than half the time that I would usually get to prepare one character.
"The other thing that was interesting is that, apart from Kevin, all the other personalities that lived within the same body, they had a different kind of genesis than you or I. You know, you or I are born because our parents did the do, but they're born out of a necessity to perform a pretty specific function within the larger group. That's because they each personify a certain quality or qualities that Kevin has," McAvoy said.
"That worried me but then as soon as Kevin came in the room as Patricia, I was like, 'Ah, right! I'm gonna have fun with this.'
"It's not just sensational, but something that could be hopefully intriguing and compelling in a way that isn't just edge-of-your-seat nerves. And as an actor there was also the opportunity to flex many muscles and employ all the dexterity you can muster," he said.