Cons 'used as extras' on A Prophet
Speaking to the BBC Jacques Audiard said that when he first visited a prison to show one of his films, he was "appalled" by what he saw.
The film tells the story of young Arab man Malik, played by Tahar Rahim, who becomes a mafia kingpin after being sent to prison.
"He's a small guy so he finds out he needs to use his brains instead of his muscles in order to survive. That's what he's doing at the start - just trying to live," Mr Audiard said.
The director revealed that using ex-convicts as extras on set was beneficial.
"They knew what to do, how to walk, I didn't have to direct them, in a corridor they knew how to look around and how to look at each other, he revealed.
A Prophet, which won the Grand Prize of the Jury at Cannes, is released in the UK today (January 22nd).