Robert Wieckiewicz portrays the man Leopold who hides the Jews. Leopold is a sewer maintenance worker who knows the sewers better than anyone, as well as early on is just a small time thief. At first he does not hide the Jews who take refuge in the sewer for any sort of noble reason. He in fact only hides them because they agree to pay him for his services and that it is all. He continues to hide them even after they run out of money and stop paying him, and even after it becomes far far more dangerous for him to keep their existence under wraps.
Robert Wieckiewicz portrays Leopold in a rather blunt fashion as just a man trying to make some money at first. He really does not try to make him particularly devious in his thievery, and there is not really any doubt shown in him just really the habit of going about his business to survive during the war. Wieckiewicz really does not depict all that much of a transition in his character, even though Leopold becomes completely selfless after awhile. This is not really a problem though as it is an interesting way to portray Leopold's motivations, and really Wieckiewicz takes a realistic approach to his entire characterization.
Wieckiewicz really plays it as Leopold merely doing what he is doing because simply that is what is right to do. Although it is true he keeps going even after doing it for the money at first their are not the moments of revelations like say there were in Liam Neeson's performance in Schindler's List, but Leopold is very different from Oskar Schindler. As he goes from doing it for the money to doing for the good of the people he treats both with the same workman like approach. Wieckiewicz stays true to the character by doing this actually, and really is showing that Leopold was simply a good man all along.
Leopold has a rather strange relationship with his Jews as portrayed by Wieckiewicz. He never does seem to really connect with them and is always distant protector of them. Even when he is telling them they need to move or bad news Wieckiewicz presents it as Leopold still just mostly doing his job. He shows that Leopold never does connect with them and always is an outsider to them even by the very end of the film. Wieckiewicz is actually effective again because he very realistically shows that Leopold never feels in the same position as they are.
Throughout the film Wieckiewicz is very to the point in the role just carrying through the scenes quietly by honestly portraying Leopold's journey through hiding the people. He is very consistent save for the very end when Leopold has believed they have all died, and Wieckiewicz shows the horrible distress he undergoes over believing the loss of the people. The other scene is at the very end where he joyously tells the people to come out. It is a moving scene as he shows Leopold's genuine pride and happiness at bringing the group of people alive through the war. This is not a great performance by any means as the role is relatively simple, but Wieckiewicz nevertheless gives a good realistic performance.