"We did spend time at Pine Street Inn homeless shelter, where Nick Flynn, who wrote the memoir, had worked. It was interesting to see him kind of blend into the woodwork, which he did very well," said film’s director Paul Weitz.

"He had a cap pulled over his eyes and he pretended to have a cold so he could cover up his face.

"There was a very odd moment where somebody who worked at the shelter said, ‘Oh, maybe Bob De Niro could play me!’ He had heard something about how we were doing a movie and he did not realise that Bob De Niro was standing seven feet from him.

"Bob likes to play real people so he was trying to get a sense of the reality there. In the shelter he observed the people staying there and their body language wasn’t downtrodden. They had erect posture and didn’t want to appear to be marks and get robbed or beaten up so that’s how he appears in the film," Weitz added.

De Niro also went to New York’s Financial District in disguise to get a sense of what it was like to be an invisible homeless person in a busy place like this, reports dailystar.co.uk.

"We chose different places where we hoped people would ignore a huge movie star. So we went down to the Financial District when everyone is arriving at work so they don’t give a damn; they’d pretty much run anyone over to get to work," he said.