Colin Farrell's new crime thriller "Dead Man Down"opens this Friday.  And to promote it, an ad agency in New York pranked a bunch of people . . . by staging a fake MURDER in an elevator, and letting them walk in on it. When the elevator doors opened, they saw a guy STRANGLING another guy with a rope, and a lot of them just screamed and ran away.  But a surprising number actually tried to stop it. And one guy just stood there taking pictures with his cell phone.