I think that a fundamental premise of both works is what happens when the heart decides how the individual should act in the conflict between desire and duty.  Certainly, Zhivago and Karenina are on the outside of the social order with the choices they make or how the social order is constructed.  In this light, both works place their focus on the outsider as a result of choice and configuration.  For both protagonists, their own choices are about as much to blame as the worlds in which they live.  I think that it might be interesting to also focus on how infidelity for a man is perceived as different for a woman.  Both protagonists succumb to the idea of infidelity and pursuing another outside the realm of marriage.  Yurii's infidelity is not perceived or understood in the same manner as Anna's.  There seems to be a disconnect of sorts between how infidelity is understood with a woman in Russian society than a man in Russian society.  Certainly, one of the main ideas of both works is the role of social and political notions of the good playing a large role in how each protagonist acts and lives.  Yet, underscoring this is the idea that within such structure, the individual makes active choices in the collision between equally desirable, but ultimately incompatible courses of action.  Into this paradigm, both works develop their protagonists.