The Doctor is a wealthy, philosophical sort. He is interested in higher thought, but at the same time, is unable to really oppose anyone, and has never really faced hardship. Ivan has faced hardship, and is a bit mad. His philosophies come out of that. As a result, Ivan is much angrier, sharper, and more cynical than the doctor. He is mistrustful of people. Their dynamic can be characterized as the nice one (the doctor) who won't give up on the other person, and the angry, cynical one (Ivan) who slowly begins to trust the nice one despite himself. Of course, Ivan seems to be making a big impression on the doctor as well. Ivan is also more practical.