Rollo is a 35-year-old British writer of Western novels who writes under the pseudonym Buck Dexter. He travels to post-war Vienna at the invitation of his childhood friend. Impulsive and prone to drinking, he refuses to accept the police's assessment of Lime as a criminal. He acts as an amateur investigator, trying to clear his friend's name and figure out the conflicting accounts of the car accident.

Calloway is a cynical British police officer stationed in the divided, ruined city of post-war Vienna. He narrates the events surrounding Martins' investigation. He operates with calculated pragmatism and firmly believes Harry Lime was involved in severe criminal activities. He attempts to warn Martins away from the city but eventually uses the writer's amateur snooping to his advantage.

Harry Lime is an English expatriate and Rollo Martins' closest childhood friend who ostensibly works with war refugees in Vienna. He dies in a sudden car accident right before Martins arrives. According to police reports, he is suspected of running a dangerous racketeering operation in the city. He inspires fierce loyalty from both his girlfriend and his oldest friend.

Herr Koch is Harry Lime's neighbor who witnessed the fatal car accident. He provides Martins with an account that directly contradicts the stories told by Lime's close associates, noting that Lime died instantly. He wants to avoid official police involvement and becomes anxious when Martins suggests pressing the matter.

Anna is a young actress and a Hungarian refugee living in Vienna under false papers to avoid deportation. As Harry Lime's grieving girlfriend, she initially shows little interest in investigating his death. She eventually cooperates with Martins when she realizes that Lime's associates have been providing inconsistent accounts of the accident.

Kurtz is an associate of Harry Lime who wears a conspicuous toupee. He claims to have been present at the car accident and reports that Lime gave him specific instructions before dying. He actively tries to dissuade Martins from looking into the circumstances of the accident, warning him that it would be highly unwise.

Dr. Winkler is Harry Lime's personal physician who was present when Lime supposedly died. He is exceedingly reticent and offers very little information about the event. He maintains an obsessively tidy home filled with religious artifacts, including a specific Jansenist crucifix that hints at his Calvinist-leaning Catholic beliefs.

Crabbin is an earnest representative of the British Cultural Relations Society. He mistakenly believes Rollo Martins, who travels under the pen name Buck Dexter, is the highly esteemed novelist Benjamin Dexter. He enthusiastically books Martins into a hotel and arranges for him to speak at a literary event, completely unaware of the mix-up.

Cooler is an American friend of Harry Lime who comes across as warm and highly sociable. He was allegedly present at the car accident and frequently emphasizes the concept of duty. He attempts to ply Martins with drinks and uses subtle persuasion to convince him to drop the amateur investigation.