The mood of the novel is largely exciting and suspenseful, as Bromden watches changes come over the hospital after McMurphy's arrival. For example, in Part Two, Bromden eagerly anticipates McMurphy's next move and wonders how Nurse Ratched will react:
I was certain that any minute he was going to do some crazy thing to get him up on Disturbed for sure. I’d seen the same look on other guys before they’d climbed all over a black boy. I gripped down on the arm of my chair and waited, scared it would happen, and, I began to realize, just a little scared it wouldn’t.
Things have been stagnant on the ward for years. Nurse Ratched holds the "Disturbed" ward over the patients' heads to get them to submit to her will, and no one ever does anything. McMurphy is the first patient in a long time who does not let Ratched frighten him into submission. Bromden describes being "scared [an altercation] would happen" and also "just a little scared it wouldn't." If McMurphy is tackled and sent to Disturbed, things might end poorly for him. It will also be another piece of evidence that there is no hope to be had in the hospital. On the other hand, if McMurphy doesn't bait Nurse Ratched into an altercation, everything will remain soul-crushingly the same as it has always been. The reader shares in Bromden's suspense, fearing for McMurphy and hoping for the revolution he is about to bring about.
There are somber moments throughout the novel. For instance, Cheswick and Billy Bibbit's deaths both serve as reminders that there is not a happy ending available to every patient in the hospital. McMurphy's lobotomy is the ultimate tragedy of the novel. However, while Kesey could leave the reader with a sense of despair, he instead turns McMurphy into a martyr who helps Bromden look towards his future with shaky excitement. The novel has plenty of searing critiques of mental institutions and the society that keeps them running, but it nonetheless insists that there is something better out there for the characters and for many real-life patients of mental institutions.