Start with Bromden's initial summary of the nurse, found here: http://www.enotes.com/one-flew/part-1-summary
That gives a strong overview, and explains the inmates' fear of her.
Late in Part I, she refuses to turn on the television despite the inmates' unified protests. This shows resolve...and her screams show she is not balanced.
Of course, the shock therapy in Part 4 is the ultimate example.
The are a myriad of examples of this; here are a few:
- At the beginning of Part 1; Bromden refers to nurse ratchet as 'big as a tractor' in his introductory narration. This metaphor alludes to the mechanical power and persistence of Nurse Ratchet.
- Bromden also states that she 'sit[s] in the center of this web of wires like a watchful robot' The web is referring to the hospital and alludes to the fact that he peceives her as omnipotent and omniesceint.
- Early in Act 1, Bromden described the subtle manipulations of Ratchet. This is to 'be calm and wait, wait for a little advantage, a little slack, then twist the rope and keep the pressure steady. All the time.' This indicates the strength and subtleties with which she operates. It warns that though she may not be outwardly aggressive she is tenacious and never lets off.
- In act two after the her outburst of anger, she quickly composes herself. in a staff meeting shortly after she states 'We have weeks, or months, or even years if need be'. She is referring to the fact that McMurphy is committed. This proves her tenacity that even after a severe loss she is able to access a situation and find its merits. She does not fold, continuing to battle until she comes out as victor.