Sorry for replying so late. Haven't checked the thread in quite some time.
Well I know this is to a large part intended by Kubrick: The Duella character seems always totally emotionless and without any motivation. He is basically blank for the entire screen time. For me it definitely looks like the guy just can't act. It takes acting chops even to maintain a blank face convincingly. I have seen Duella later in a minor film, much older (can't remember which one it was), he had a pretty straighforward talking role - my feeling was right: the guy can't act. Please correct me if I'm wrong.