When something wasn’t quite pushing it for Martin Scorsese while filming a pivotal scene in “The Departed,” Jack Nicholson unexpectedly pulled a real weapon on co-star Leonardo DiCaprio to bring the edge. “The first thing Jack did was sniff the glass and say, ‘I smell a rat’ . . . And then he pulled a gun on him,” Scorsese told Richard Schickel for his upcoming book, “Conversations With Scorsese.” “He didn’t tell me he had a gun. It was great . . . we took a lot out, but Leo’s reaction is real-time.” He said, “I still get chills,” when Nicholson says, “I smell a rat.” — “It’s so real to me.” Scorsese’s love of spontaneity started early. As a budding director, he tricked a young Harvey Keitel, who was auditioning for a role in his student film “Who’s That Knocking at My Door,” into an argument with comic Bill Minkin to see how he would react: “They got into a big argument. I thought it was great! That was the audition I set up, but I neglected to tell Harvey. And Harvey got so mad at me.”