Chief Bromden's apparent paranoia is expressed in the first lines of the novel. He is obviously frightened.
"I creep along the wall quiet as dust in my canvas shoes, but they got special sensitive equipment detects my fear and they all look up." (Kesey, pg. 9)
A patient in the hospital for many years, everyone thinks that he is deaf and dumb because he does not talk.
But, at the end of Chapter One, the reader understands that it is Chief Bromden who is the narrator of the story. He sounds very sane when he says:
"I been silent so long now its's gonna roar out of me like floodwaters and you think the guy telling this is ranting and raving my God; you think this is too horrible to have really happened, this is too awful to be the truth!" (Kesey, p.13)
The story suggests that Chief Bromden was probably sane when he went into the hospital, his mental illness is never diagnosed. But he retreats further into silence after many, many electric shock treatments, and being put into isolation without food. His condition is intensified by the treatment that he receives in the mental ward.
How mentally ill could he have been if McMurphy brings him out of his silence and helps him recover.
So, Chief Bromden may be sane all along, just withdrawn and feels that no one listens to him, so why talk, a feeling he has had for a long time, since he was a young boy.