Even though we are not in their positions, I believe that we can offer insights into the characters' actions in The Book Thief.
There is a difference between "judging" and giving our own perspective. When we judge, it is a hardened statement as to right and wrong. Judging silences any further discussion because of its finality. This might be difficult to do because we are not in the character's predicament. For example, it is difficult to judge someone's actions during the Holocaust as "right" or "wrong" because we will never know what it was to be placed in such a situation.
However, it is possible to render our own perception about individual actions. When Hans struggles with the moral implications in joining the Nazi party, he is trapped between two equally desirable, but ultimately incompatible courses of action. In this interest, his choices are to stand against intolerance or protect his family. While we hope we are never placed in such a situation and might never understand the full implications of what he experienced, we can discuss what is involved in such a decision. We are able to examine the calculus that guides how he proceeds, and even offer our own insight as to the values we hold dear. While we should not steadfastly judge, we can speak and explore the complexity in enormous decisions. Doing so enables us to better understand the choices people make.
If we presume that we cannot speak about a specific character because we are not there, we tend to fall into the same silence that benefits those who support tyranny. Complexity and open discussion is the best way to stop the forces that seek to remove individual voice. I believe that there is a way to offer our own voice without condemning someone else's. It is in this plurality of voice and embrace of tolerance that I would suggest we are able to speak about a character's moral choices even if we are not in their exact situation.