Well, Richard Parker really represents Pi's subconscious. A major idea in this novel is that we all have a duality, a dual nature. We have the life we live and we have the life we want to live-- and existence happens somewhere in the middle. Reality for Pi, the harsh, cruel, and gruesome nature of humanity, is not his reality or his truth, so he creates his own. He creates a world that makes sense, a world with a good and evil dichotomy that he can comprehend and live with. Richard Parker is the manifestation of his fears.
I can't answer for most readers; however fear is an important aspect of the relationship for both the tiger and for Pi. The tiger was probably not fearful in the same way as Pi. For the tiger it would have been more instinctual. Pi made himself the Alpha animal and asserted his dominance. This something the tiger would have understood and responded to. So in this way, he is fearful only because his nature tells him to keep his place in the hierarchy. Pi, on the other hand, knew he must remain the dominant 'animal' and therefore was afraid of what might happen should his dominance slip. Were that to have happened, the tiger would have become the Alpha animal and Pi would have been lost.