It has been hailed as the most realistic screen depiction of the mafia ever made, a gritty affair with an almost documentary-like quality that has been tipped for Oscar glory after winning second prize at Cannes. But if the Italian film Gomorrah does pick up any Academy Awards, its director, Matteo Garrone, may be without a cast member or two to cheer him on. Since the movie opened in Italy, two have been arrested and a third is being investigated over allegations of real-life mafia involvement.

To maximise realism, Garrone cast a number of locals from Naples's most crime-ridden suburbs in several roles. Bernardino Terracciano, 53, who plays Uncle Bernardino in the film, was seized on Saturday on suspicion of extorting protection money and having ties to the Casalesi clan, part of the Camorra mafia, which has terrorised small towns north of Naples this year in a campaign that culminated in the shooting of six African migrants on September 18.

His arrest follows that of Giovanni Venosa, who plays another boss in the film and who was detained in July on suspicion of playing the same role in real life. A third actor plucked from the streets of Naples to play a hit man, who is seen in the film bursting into a sun-tanning parlour and opening fire, is being investigated for drug dealing, the Italian newspaper Il Giornale reported yesterday.

Garrone has said he approached the film as if it were a "war documentary" where "the director is invisible" and which works "on an emotional level, not a rational level". He cast theatrical actors alongside known cinema stars like Toni Servillo, combining them with a handful of Neapolitans with little or no acting experience hired on the spot, a formula he used in an earlier film set outside Naples, L'Imbalsamatore, or The Embalmer.

One actor in Gomorrah, a fruit and vegetable seller in real life who was cast as an aspiring mobster, made his first ever trip outside Naples to help promote the film in Cannes. After deciding to use Neapolitan dialect for the dialogue, Garrone then added subtitles in Italian for the film's domestic release.

One murder scene filmed in a rough Naples neighbourhood was videoed by locals using mobile phones and circulated, prompting a police investigation. "I met bosses who showed me that footage on their cell phones and told me that in addition to carrying out hits, from now on they would film them too," Garrone said.

The mob has long provided a rich seam for filmmakers, and like mobsters around the world the Casalesi have been inspired by on-screen mafiosi, notably Al Pacino in Scarface. Walter Schiavone, a senior boss, gave an architect a video of the film and told him to build a replica of the villa in which Al Pacino is murdered by rivals. Now standing empty, the villa appears in Gomorrah.

Terracciano's appearance in the film however marks a new trend for mobsters. No longer content to sit back and study celluloid gangsters like Pacino, suspected real life bosses are upping the ante by starring in the films themselves.

To make Gomorrah, Garrone was happy to take tips from locals when he filmed in some of the most dangerous neighbourhoods in Naples. "When we shot these scenes, they were always looking on", he has said, "providing advice and actively participating."

Based on the best selling book by the Italian journalist Roberto Saviano, Gomorrah describes the vice-like grip of the Camorra on Naples through drug dealing, building contracts, prostitution and the dumping of toxic waste in fertile agricultural land, as well as control over the city's rubbish collection.

Saviano, who is now under police protection, has continued to campaign against the rule of the Casalesi clan, the most feared grouping within the Camorra, which has this year launched a campaign of killings, prompting the dispatch this month of 400 extra police and 500 soldiers to the clan's fiefdom around Casal di Principe. Yet killings have continued, despite the arrest of suspected hit men.

"The plan has been to terrorise businessmen and family members of turncoats as well as discouraging future turncoats," one of those arrested, Oreste Spagnuolo, told police, Corriere della Sera reported yesterday.

Despite the erection of army roadblocks and a wave of arrests, the Casalesi were suspected of last week ransacking the flat of a local journalist, Rosaria Capacchione, who has documented the clan's empire and who also lives under police escort.

Made men

Gomorrah is not the first time cast members have fallen foul of the law in real life. The Sopranos had a litany of cases in which actors were charged with criminal offences. In 2006 Louis Gross, cast as bodyguard to mob boss Tony Soprano, was charged with criminal mischief for an alleged break-in. He cut a deal with prosecutors. Another actor, Lillo Brancato Jr, goes on trial next month charged with the murder of an off-duty police officer in 2005. Vincent Pastore, "Big Pussy" Bonpensiero, pleaded guilty in 2005 to attempted assault on his girlfriend and was sentenced to community service. Robert Iler, Soprano's rebellious son, pleaded guilty in 2001 to mugging two youths and stealing $40; he got three years' probation. Tony Sirico, Paulie Walnuts, had before the series been arrested 28 times, and was sentenced to four years for holding up a nightclub in the 1970s.

Caroline White