In the first movie where Al Pacino and Robert De Niro starred together, The Godfather: Part II (1974), they both played gangsters. In their second movie Heat (1995), Pacino was a cop, and DeNiro was a gangster. In this, their third movie together, they are both cops. This is also the first movie where they have extended interaction. They never shared a scene in The Godfather: Part II (1974) and in Heat (1995) they only had two scenes.
When Robert De Niro, Al Pacino, and Brian Dennehy are speaking about the murder investigation of "Bob Brady", De Niro says, "Seriously, lieutenant, how far do you want us to go with this Rambo murder?". Dennehy was the main antagonist, "Teasle" in Rambo: First Blood.
The initial cut of Righteous Kill ran for two hours, but after a rather negative test screening, Nu Image hastily re-cut the film down to its slender 101 minutes to make it more fast-paced and action-orientated. Snippets of some of the deleted scenes can be seen in the trailer.
While staking out Turk as a potential suspect, Riley and Perez talk about Turk's marksmanship scores, remarking about his skill and accuracy while saying "Even Berkowitz but a hole in the Corinthian leather." This is a reference to David Berkowitz, also known as the Son of Sam, who was a serial killer who targeted young women with dark hair. In the Spike Lee movie, Summer of Sam, John Leguizamo (who plays Perez) stars as a resident in a movie about the infamous Son of Sam, real name of David Berkowitz, and his effect on the citizens of a small neighborhood in New York.
The trivia items below may give away important plot points.
Pacino and DeNiro's characters are only ever reffered to by their nicknames Rooster (Pacino) and Turk (DeNiro) through the majority of the film, as to not give away the twist ending that the video "confession" being read by Turk where he reveals that David Fisk aka Rooster is responsible for the ongoing murders that he and Turk are investigating.
After DeNiro's video statement revealing that "there is a bullet lodged in his torso that was meant for me" there is a scene at Carla Gugino's apartment with DeNiro shirtless showing a surgical scar on his back, hinting that he is in fact the one with the bullet in his torso, not Pacino