I think that the rape scene is one of the most important scenes in which Amir and Hassan interact.  It is here where the emotional crux of the novel takes place.  Amir and Hassan interact in a specific moment where both have different experiences and reactions to it.  Amir is struck with paralysis while Hassan is brutally sodomized, one forever wondering if his friend will help him in a moment of pain while the other wondering if his friend recognizes his own moral cowardice.  It is an instant where the interaction between both changes the development of the novel.  The scene preceding it with Hassan chasing down Amir's kites at the kite festival is also important because it shows the extent of friendship between both and also because it shows the extent that both believe in one another and how there is a spirit of community and solidarity between them, undermined by the very next scene in the alley.  I would say that another important scene of interaction between both would be after Hassan has been killed.  His letter to Amir is one in which the full recognition in both the political reality that has ravaged Afghanistan as well as the condition of the eternal friendship that Hassan has towards Amir is evident.  It is an interaction that takes place beyond the grave, speaking to the power of a friendship "a thousand times over."