The main character is Nicholas Salmanovitch Rubashov, a man in his fifties whose character is based on "a number of men who were the victims of the so-called Moscow trials," several of whom "were personally known to the author". Rubashov is a stand-in for the Old Bolsheviks as a group, and Koestler uses him to explore their actions at the 1938 Moscow Show Trials.
Secondary characters include some fellow prisoners:
- No. 402 is a Czarist army officer and veteran inmate.
- "Rip Van Winkle", an old revolutionary demoralised by 20 years of solitary confinement and further imprisonment.
- Hare-Lip, he "sends his greetings" to Rubashov, but insists on keeping his name secret.
Two other secondary characters never make a direct appearance but are mentioned frequently:
- No. 1, representing Joseph Stalin, dictator of the USSR. He is depicted in a widely disseminated photograph, a "well-known color print that hung over every bed or sideboard in the country and stared at people with its frozen eyes."
- Old Bolsheviks. They are represented by an image in his "mind's eye, a big photograph in a wooden frame: the delegates to the first congress of the Party," in which they sat "at a long wooden table, some with their elbows propped on it, others with their hands on their knees, bearded and earnest."
Rubashov has two interrogators:
- Ivanov, a comrade from the civil war and old friend.
- Gletkin, a young man characterised by starching his uniform so that it "cracks and groans" whenever he moves.
