According to the Italian daily newspaper Corriere della Sera, the film was screened at a Naples prison where inmates in jail for activities related to the Gomorra -- Naples' version of the Mafia -- recognized one of their own among the film's cast.
The actor, credited in the film as Giovanno Venosa, who is reported to be a wanted organized crime figure with a passion for acting, ended up as part of the no-name cast that director Matteo Garrone selected for the film. He tried out in an open audition and won a role.
Police said they easily tracked him down and that he was placed in prison in the northern Italian city of Modena, near Bologna.
"We would have never known Venosa was who we were looking for if he had not been recognized by the inmates who saw the film," a spokesman for the Carabinieri in Naples said in a telephone interview.
In the film, the character played by Venosa killed two teenagers. Police did not say which crime he has been accused of in real life.