It is clear that, as the novel progresses and Susie's father becomes ever-more frustrated by his increasing certainty that George Harvey committed the murder but without any evidence to prove it, he becomes more and more isolated and slightly deranged. It is clear that his inability to let his daughter go and move on results in the estrangement that his marriage suffers. Susie is shown to be incredibly protective of her father as he goes through this period of his life. Note how she describes a scene in her father's study:
In his low green easy chair he now felt the most comfortable. It was where I often saw him sleep. The room like a vault, the chair like a womb, and me standing guard over him.
Susie explictly reverses the normal father-daughter dynamic, as she becomes the protector over her father and longs to see him move past this stage. The reference of the room and the chair being like a "vault" and a "womb" respectively shows how Jack seeks protection and safety from his study. The way in which Susie is shown to be her father's protector demonstrates her own sadness and pity at how her father is showing his inability to cope with her death.