Throughout the entire film there really is a lack of emotion being shown by Theo from the death of his loved ones to the very end. I believe Clive Owen portrays his character in this way to really emphasize the fact that there is really a lack of hope in the world. Even when Theo is given the responsibility of transporting Key across the most dangerous terrains he consistently shows his lack of emotion. Perhaps this could be seen as character development or just poor character portrayal, that decision is soley up to the viewer. Or it could be that the director wanted it this way and wanted him to resemble the lost cause of the world.
Although I agree that Theo can seem emotionless at times, I feel that Theo is only trying to conceal his emotions at the times where he must keep his cool. Theo’s emotions do come out however when burying Julian. During this time Theo begins to break down by the tree and doesn’t want the others to see him crying. Since he is now on the run from the law, he can’t allow his emotions to get in the way. Again, after the death Jasper, Theo doesn’t let his emotions get in the way as he needs to escape Luke and the others. In this scene he is clearly angry but doesn’t really have the time to mourn the death. Because he is playing the hero role, Theo can not allow his emotions to make him vulnerable.