In Starred Up, Mendelsohn plays a rather complex figure as Neville is somebody who acts as a rather uncompromising mentor to his son yet is only desperate to get his son out of prison away from the "big boys." But Mendelsohn showcases Neville's desperation through sudden shifts in his eyes and body movements. Take for example, this scene:
Notice how in the beginning, he is all shaky and clutching his hands. Then as he starts spewing out cusses, when you look closely in his eyes at about 0:32, you can see the scared anxiety behind that rough exterior. Yet Mendelsohn never dwells on the melodrama. But when the camera cuts back to a farther distance, the therapist, played by Rupert Friend, remind Neville how he wasn't there for his son as Eric suffered abuse in the past. Suddenly, Neville turns on a dime and goes on an angry tangent. Even from a distance, you can witness his sudden switch in emotions, ranging from anxious and scared to guilt ridden and insecure. That is thanks in large part to how Mendelsohn uses his facial and bodily shifts to showcase Neville's complexity even in just one scene like this one.
So those are my quick thoughts on Ben Mendelsohn's performance as Neville in Starred Up. Whether you agree or disagree on my thoughts and would like to offer your own analysis on his work, please feel free to share your own thoughts in the comments section. Thanks for reading!