In a heavily circulated bootleg tape of Dean Martin and Jerry Lewis recording radio spots for their 1953 Paramount film, “The Caddy,” Martin mangles the ad copy they’ve been given and mispronounces one piece of hyperbole as “righteous” instead of “riotous.”

“‘Righteous’? Where the f— do you see ‘righteous,'” Mr. Lewis demands, ad-libbing. “What is this, a religious picture?” Both actors then descend into an exchange of barbs even less printable in a family newspaper.

Each time I’ve read or heard the title “Righteous Kill,” the new film boasting the lengthiest on-screen pairing of Al Pacino and Robert De Niro, that exchange has come to mind. As The New York Sun’s S. James Snyder observed last Friday, during the last dozen or so years, Messrs. De Niro and Pacino have charted parallel courses away from universal acknowledgment as the finest American actors of their generation to self-parody in a series of misguided starring vehicles that increasingly showcased nothing more than their eagerness to get paid. Would this new film, inauspiciously directed by Jon Avnet, helmer of Mr. Pacino’s last orgy of dinosaur-size scenery chomps, “88 Minutes,” be an inadvertent comic embarrassment and yet another irrelevant feature-length outtake from both actors’ careers?

The answer is yes. And no. As a comeback vehicle, “Righteous Kill” is not exactly built for comfort or speed. Longtime NYPD detective partners “Turk” Cowan (Mr. De Niro) and “Rooster” Fisk (Mr. Pacino) preside over a convoluted whodunit concerning a vigilante killer whose taste in low-life criminal victims is so refined that it becomes increasingly clear whoever is pulling the trigger has a badge himself. “It’s a cop, it’s a cop, it’s a cop, it’s a cop,” crows Detective Perez (John Leguizamo), who, along with his partner, Detective Riley (Donnie Wahlberg), has been drawn into the case.

But which cop? Is it Turk, who already has a secret or two he’d like to keep to himself? Is it Rooster, a chess-playing philosopher with a crazy collegiate mop of hair to complement his alternately cerebral and sarcastic demeanor? Could it be Detective Corelli (Carla Gugino), Turk’s kinky sometime bedmate and apparently the only forensic investigator in the NYPD?

What starts out as a kind of Agatha Christie’s “Ten Little Indians” in blue quickly narrows down to five, then three, and by the film’s end, one. But, as is usually the case in these kinds of sagas, the divulgence of the real killer’s identity and motivations are, by the film’s climax, old news, as anyone even slightly engaged by the proceedings will have at least considered the real culprit among a half-dozen other possible scenarios.

Given more to absorb in the way of original dialogue, procedural detail, or evolving characterizations, that wouldn’t be a problem. But the sole bid for storytelling originality in “Righteous Kill” is an ostensible video confession interpolated throughout the film that’s frankly not very original and collides head-on with a similar scripted device involving sessions with a police psychologist. At times it is difficult to sort out whether “Righteous Kill” takes place in a single weekend or during the course of a year or more.

Thankfully, even though the film’s script, by “Inside Man” writer Russell Gewirtz, plays to the cheap seats, Mr. Pacino keeps his post-“Scarface” tendency toward implosive, bizarrely out-of-touch histrionics under wraps — at least, until the last reel. While Rooster is still a far cry from the hollow-eyed, haunted characters that made Mr. Pacino both a box-office success and a critical favorite in decades gone by, “Righteous Kill” does contain perhaps his most subdued performance so far this century.

Mr. De Niro, who has lately squandered his somber on-screen gravitas by appearing frozen in the same strained, wincing, dyspeptic semi-smirk whether playing in a comedy or a drama, for the most part remains nearly expressionless. He pouts, frowns, and scowls with such dogged closed-mouth solemnity that he appears on the verge of evoking a middle schooler returning to class after a summer spent at the orthodontist.

In their primes, Messrs. De Niro and Pacino could have played these parts in their sleep. Now well out of their prime, they appear in some scenes in “Righteous Kill” to be doing just that. But their heyday was one in which a film this perfunctorily written and directed would never have been offered to them in the first place. If only they’d have shown the nerve to switch their stock roles midway through the film. Their past work indicates that they surely would have the skill to pull it off.