Jack Nicholson pulled a gun on Leonardo DiCaprio while filming The Departed and it gave Martin Scorsese 'chills'

Jack Nicholson went way further than improvising during a scene with Leonardo DiCaprio on the set of The Departed - he pulled a gun on him without telling director Martin Scorsese it was coming, and asked the prop master to hide a fire extinguisher nearby for good measure.

The bar scene sees Nicholson's mob boss Frank Costello grill DiCaprio's undercover cop about a 'gnawing, teething f-ing rat', and it originally wasn't written with a weapon in it at all.

According to Nicholson, the idea came to him the night before a reshoot for the crime movie, which premiered 20 years ago today on September 26, 2006.

"There wasn't any gun in the script," Nicholson explained in an interview with Variety.

"We had shot the scene the night before but Marty said he had a light schedule the next day and wanted to try a few more takes. I wanted to come up with something different, so I asked the prop master to hide a gun on the set, and to bring a fire extinguisher as well. The look on his face when I asked for that fire extinguisher was priceless."

(L to R) Actors Jack Nicholson, Mark Wahlberg, Leonardo DiCaprio and Matt Damon all starred in the Martin Scorcesse classic The Departed (Photo by Evan Agostini/Getty Images)

DiCaprio was caught off guard by the gun

The gun itself was a working prop, but Nicholson's decision to use it was entirely unplanned. Scorsese, who had no idea what was about to happen, later described the moment in the book Conversations With Scorsese, written by Richard Schickel. "The first thing Jack did was sniff the glass and say, 'I smell a rat'... And then he pulled a gun on him," Scorsese recalled.

DiCaprio's shock on screen wasn't acting. "He didn't tell me he had a gun. It was great... we took a lot out, but Leo's reaction is real-time... I still get chills... It's so real to me," Scorsese said.

Nicholson's role as Costello wasn't even part of the original pitch.

The Departed is Scorsese's remake of the 2002 Hong Kong thriller Infernal Affairs, in which the equivalent character barely featured. Nicholson revealed he initially passed on the project altogether.

"I actually turned the movie down the first time it came to me," he said, "because the character didn't really exist. But Leo and Marty talked me into it. I guess you can say I was attracted to the company."

Jack Nicholson played mob boss Frank Costello in Martin Scorsese's 2006 film The Departed. (Photo by James Devaney/WireImage)

The character was then built from scratch alongside Scorsese and DiCaprio.

"Marty is very free with his ideas, and very receptive to yours. We built this character layer by layer, until we had something that fit inside a great genre film, but also pushed the envelope until the movie becomes almost operatic," Nicholson added.

Critics agreed the improvisation paid off. Variety's Todd McCarthy called the performance 'theatrical in an entertaining way', while the San Francisco Chronicle's Mike LaSalle described it as 'a psychologically detailed portrait of a twisted, old murderer'.