When Martin Scorsese’s Taxi Driver premiered in 1976, it instantly put Robert De Niro’s Travis Bickle and the alienation felt in 1970s New York firmly in the spotlight.

Above all, one moment has defined the film in popular culture: the scene in which De Niro stares into a mirror, practicing confrontation, muttering “You talkin’ to me?”

The line has been parodied, quoted, and referenced for decades, but what many don’t realise is that it was never in the script. It was completely improvised – the product of a spur-of-the-moment idea between Scorsese and De Niro during the final days of filming.

Speaking at a reunion for the film’s 40th anniversary at the Tribeca Film Festival in 2016, Scorsese revealed how the now-famous exchange came to be.

“There was no dialogue, I believe, in the scene, and I remember saying, ‘Can you say something to yourself? In the mirror?’”, he explained.

It was the last week of shooting, a difficult period for the cast and crew, when Scorsese decided to roll the camera on an unscripted experiment.

The director wanted De Niro to explore Travis Bickle’s mind in one of his most unguarded moments, alone in his grimy apartment with a gun in his hand. What followed became one of the most celebrated improvised scenes in cinema history.

“They locked the door to keep other crew members out while De Niro improvised,” Scorsese explained. “He kept saying, ‘You talkin’ to me?’”

“He just kept repeating it, kept repeating it… and the [assistant director] was banging on the door saying, ‘Come on, we got to get out of here.’ And I said, ‘No, this is good, this is good. Give me another minute.’”

Scorsese likened the rawness of the moment to music, saying: “It was like a jazz riff. Just like a solo.”

De Niro himself admitted he had no sense at the time that the throwaway improv would become so relevant: “You never know with any of that stuff,” he said. “You just did it.”