It’s not that the crime thriller “Righteous Kill” is spectacularly awful. It’s just thoroughly mediocre – a standard police procedural, a long episode of “Law & Order,” unremarkable but for the pairing of Robert De Niro and Al Pacino.

But really, shouldn’t we expect more from these two, considered among the greatest actors of our time? Well, we might have been justified in having high hopes about 15 years ago, before both stars had morphed into caricatures of themselves.

They shared a few moments on screen in Michael Mann’s “Heat” in 1995, and they had no scenes together in “The Godfather: Part II.” In theory, seeing them play off each other for an entire film should have been a thrill, a clash of the scenery-chewing titans. In reality, it’s hard to take them seriously, even though each actor has dialed down his all-too familiar persona: the repressed rage of De Niro, the voluminous volatility of Pacino.

Under the pseudo-flashy direction of Jon Avnet (who also directed Pacino this year in the flop “88 Minutes”), De Niro and Pacino play geriatric New York police detectives who go by the nicknames Turk and Rooster. They’re on the tail of a serial killer: a vigilante who takes out bad guys who have it coming, then leaves a poem behind at the crime scene. As the bodies pile up, the evidence suggests that a cop is the one pulling the trigger: The script from Russell Gewirtz drags us down one obvious avenue for most of the film, only to leave us with a gimmicky, obvious twist at the end.

Gewirtz’s screenplay for Spike Lee’s suspenseful, darkly funny “Inside Man” was far more clever. Whereas that film was skillfully paced and plotted, “Righteous Kill” meanders from one shooting to the next until the climactic, bombastic confrontation between our two hammy stars at the end. Along the way, Avnet shows us chess pieces and bullet casings falling as groan-worthy metaphors. (He does make the daily rhythms of New York feel authentic, however.)

Carla Gugino seems wedged in as a crime scene analyst who becomes suspicious of Turk, her longtime quasi-boyfriend. She’s also into rough sex, a gratuitous, borderline offensive detail. Brian Dennehy is solid, as usual, as Turk and Rooster’s sarcastic lieutenant.

And rapper Curtis “50 Cent” Jackson is as stiff and monotone as ever, playing a supposedly charismatic drug dealer. Apparently, he hasn’t taken any acting lessons since starring in 2005 “Get Rich or Die Tryin”‘ – and in that movie, he essentially played himself.

Film focus:

WHAT: “Righteous Kill”

RATED: R for violence, pervasive language, some sexuality and brief drug use

RUNNING TIME: 101 minutes

RATING: 1½ out of 4 stars