Are the characters in the novel inherently powerful (or powerless), or is power the result of a person’s position in life (such as being a nurse versus being a patient)?

That's a good question. Power is at the core of Ratched's existence. She has traded her sexuality and her humanity in for power. The patients, except for McMurphy, have traded in their uncertainty and fear of the outside world to be emasculated and controlled by Ratched. This is the essential balance of power that the stakeholders have accepted. McMurphy, of course, comes along and upsets this balance.