“You’re the actor!” shouts Anna Genovese at her estranged spouse, gangster Vito Genovese, in a courtroom where he’s implausibly claiming he lacks funds to support her. “The best actor in the world! Better than Clark Gable!”

And we all chuckle. They probably all chuckled on set, too. Because the man playing Vito in “The Alto Knights” is none other than Robert De Niro, indeed one of the best actors in the world, revered in our time as Gable was in his.

It’s a cute moment and an apt one, too, because Barry Levinson’s film, which aims but fails to channel the magic of decades-old mob movies like “Goodfellas” — though penned by the same writer, Nicholas Pileggi — is all about De Niro. Actually, it’s all about De Niro AND De Niro. The man plays both lead roles, feuding mobsters Genovese and Frank Costello, in a story based on real events.

Is it a gimmick? Surely. Does it work? Well, there’s the entertainment value — this is De Niro, after all — and if you feel that more De Niro minutes are always better, then it follows that two roles are better than one. Others may feel it has a mob-themed “Parent Trap” vibe, less weighty than it should be given the obviously violent subject matter.

Also, “Goodfellas” fans may wonder if the Vito role was once intended for Joe Pesci (the movie’s been in development forever), so similar is the character to that actor’s impulsive, manically suspicious persona. On the other hand, in two brief scenes where the De Niros appear together for momentous meetings, one might be forgiven for wondering if Al Pacino was in line at some point, for a “Heat”-like moment.

Happily, De Niro relies here on makeup, and not de-aging as in “The Irishman,” though it must be said that, at times, his two characters just don’t look different enough. More importantly, “The Alto Knights,” despite its pedigree, doesn’t rise anywhere near the heights of its glorious predecessors. It is, rather, an enjoyable if choppily paced look at a relationship between two men, where unfortunately we’re arriving pretty late in the game.

There are, though, a few crackling surprises: that domestic courtroom scene; a tense, televised Senate committee grilling; and finally a climactic gathering of mob bosses in the countryside, with fabulous period vehicles parked on the lawn and sausages on the grill, that’s disrupted in comically sudden fashion.

We begin in 1957 in Manhattan. Costello, after a night partying respectably with high society, stops at his swank apartment building. “This one’s for you,” declares the nervous man who shoots him in the head in his lobby.

The shooter (Cosmo Jarvis), sent by Genovese, makes a bad mistake, as his boss will remind him later: “You gotta go SEE if they’re dead!” Amazingly, Costello survives. “I shoulda been paying more attention,” says the genteel mobster who favors diplomacy over bloodletting, narrating from the future.

We then go back in time to figure out how things got this bad.

With the help of vintage photos and footage artfully edited to include actors playing the younger Frank and Vito, we learn the two were good friends as young Italian immigrants on the streets. But when Vito got mixed up in a murder case, he had to flee to Italy — leaving cooler-headed Frank in charge of the business.

Years later, Vito returns, and wants his role at the top of the mob family. And so things go south, quickly.

The women in their lives mirror the differences between the two men. Costello’s wife of more than 50 years, Bobbie (Debra Messing, making a nice foray into drama), is a loyal partner who urges her husband to retire and leave New York — along with their adorable dogs (De Niro’s own pooches, dressed in mink coats and hats — a costume design Oscar for this canine fashion, please!). Genovese’s wife Anna (Kathrine Narducci, excellent) is a businesswoman — she owns a gay bar — whose fiery union with Vito turns disastrous.

A mere glance at Wikipedia will tell you that both Genovese and Costello ultimately died in nonviolent ways. There’s plenty in “The Alto Knights” that you won’t see there — for example, the movie’s version of just who engineered that farmland mob summit, and why.

But watching dozens of mobsters run like heck when police happen upon their gathering is a hoot, and it’s hard to beat the moment when two of them insist to suspicious cops: “We’re hunters!”

If only such witty moments were more frequent in a two-hour movie that somehow, alas, feels much longer.

“The Alto Knights,” a Warner Bros. Pictures release, has been rated R by the Motion Picture Association “for violence and pervasive language.” Running time: 120 minutes. Two stars out of four.