The multi-Oscar winning film tells the tale of an undercover cop, played by DiCaprio, who infiltrates a gang led by Nicholson's crime boss character.
One famous scene sees the mobster voice his suspicions of a traitor in his midst, and director Martin Scorsese reveals Nicholson shocked everyone on set by deviating from the script and pulling out a weapon to frighten his co-star.
And even though the sequence was later edited out, Scorsese admits he is still moved every time he sees the Titanic star's reaction on-screen.
In a new book, Conversations With Scorsese, he says, ""The first thing Jack did was sniff the glass and say, 'I smell a rat'... And then he pulled a gun on him. 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 (when he says) I smell a rat. It's so real to me.""