What is the most famous theme in Henry James.s fiction? What distinguishes him from other realist writers such as Mark Twain and W. D. Howells?

The most famous theme is "the international theme". His novels or short stories of the theme are always set against a larger international background, usually between Europe and America. They center around the conflict of the two cultures, represented by an innocent American and a sophisticated European. James is the founder of phychological realism for his psychoanalytical approach to his characters.