Within the theme of growing up, the truth becomes an important issue. A large part of being a child is discovery. The process of discovering the way the world works and understanding other people is central to maturation. In Number the Stars, the war makes it necessary for the truth to be concealed at times. As Henrik explains, knowing too much can make bravery harder and therefore put a person at risk. This, however, creates a contradiction for the process of growing up. Not knowing is usually equated with being a child. But in the story, not knowing means being able to face danger, which is a mature trait. Thus Annemarie must find a balance between demanding the truth and protecting herself from it.
Annemarie learns that judgments based on appearances are not always correct. This lesson is related to the theme of growing up, since learning to avoid judging on appearances alone is part of maturing. Particularly concerning pride, Annemarie discovers that what you see at first may not be a manifestation of what is actually so. At first, Annemarie thinks that the Rosens have lost their pride because they wear shabby clothing and make a forced escape. But she quickly realizes this is not true. Their pride is intact and is not attached to the objects they possess. The casket, too, is an instance of miscalculation based on appearance. A casket normally contains a dead person, but in this case it holds items that will help the escaping Jews to survive.
The war forces individuals to conceal their true identities and actions. In various situations, the characters of Number the Stars must pretend to be someone they are not. Ellen's desire to go to acting school is painfully echoed by the events of her own life, in which acting becomes a matter of survival. Ellen must act as if she is Lise Johansen in order to save herself. Annemarie also finds herself playing roles. While Mrs. Johansen is taking the Rosens to Henrik's boat, Annemarie takes on the role of the mother. Then she plays the part of the silly little girl when the soldiers stop her in the woods. Role-playing can also be connected to the process of growing up. Children play at being adults in order to try on the responsibilities of the grown-up world without having to assume them in reality.