In constructing each of the characters in the way he does, Hosseini uses characters' reactions to the world around them as a way to develop plot. Essentially, each of the characters believes something internally that collides with the world that envelops them. Their reactions to this dynamic is how plot and themes advance.
For example, Jalil loves Mariam. Her love for him is rooted in how much he cares for her. Yet, this comes into direct collision with the expressed opinions of his other wives. The external world holds Jalil hostage and thus the development of the plot in which Mariam can no longer love him advances. Upon this love withering, Mariam's life advances and with it, the plot. Jalil's choices between his own emotions and the world have implications that advances the plot. From this, Rasheed becomes a part of the plot. If Jalil were able to speak more in his love for Mariam, perhaps Rasheed would not have been so eagerly sought as a suitor. Rasheed's zeal and willingness to embrace the subjugation of women helps to advance the plot. His disdain for women and belief that the woman should only deliver children, namely sons, is how his relationship with Mariam advances. At the same time, when she cannot deliver a son, the plot advances with him taking Laila as a second wife in order to procure a son. Rasheed's willingness to embrace a world around him that mirrors his own belief of men being superior and women inferior helps to advance the plot. At the same time, it enables an appropriation of Taliban values within the domestic realm, something that ends up significantly impacting both women's narratives.
Tariq is able to advance the plot because he is the one man that embodies and acts upon values that are antithetical to the world around him. Tariq's relationship with Laila and even his mere presence when she believes him to be dead helps to provide some notion of hope in a world where it is strikingly absent. The hope that Tariq presents to Laila as both a character and a symbol moves the plot as it provides an alternative to what can be as opposed to what is. Hosseini's advocacy of self- determination in the face of overwhelming brutality and sadness is represented in Tariq. It is in this light where he is able to move the plot through his ability to stand for his beliefs in stark contrast to the world around him.
The notion of hope and redemption is evident in both women. Laila and Mariam both advance the plot because of their fundamental collision between their own values and the world around them. Both women are "outsiders" because their own way of thinking is not validated in the surrounding world. Their collision between what is and what should be help to develop the plot. Both women forging a bond between one another and both being there for the other drives the plot. No better is this seen than when Mariam kills Rasheed. The ultimate collision between what is and what should be moves the plot because Mariam acts upon the latter in the face of the former. Mariam drives the plot until the very end, as her executioner demands that she bow her head while she internally recites verses from the Qur'an, while Laila knows what she will name her next child. In this, Hosseini is able to construct both women acting upon their beliefs despite the disapproval of the world as a critical agent in driving the story's plot.