Violent and at times over the top, including a scene with Jack Nickolson wearing a strap-on dildo, Martin Scorcese’s new film “The Departed” opened Friday, and packs an abundance of depth in a smart crime drama.

Starring Matt Damon and Leonardo DiCaprio, the movie is, at its bear bones, a film about two moles following each others’ shadows from one violent crime to the next, one working for the police and one working for Boston’s Irish mafia.

The stress of trying to maintain their covers while trying to stay one step ahead of each other fuels the suspense. Both Damon and DiCaprio do wonderful jobs communicating the exponentially growing weight of their burdens. At many times DiCaprio looks as if he will burst into tears from the strain.

Surrounding two strong lead performances are notable showings by Martin Sheen, Alec Baldwin, Mark Wahlberg and Nicholson, who struts around in a leopard print robe and tosses a fistful of cocaine onto his bed before an implied threesome.

Scorsese’s sprawling, decadent 149 minutes of crime drama derives itself from a slick minimalist Hong Kong thriller.

The original, “Infernal Affairs,” 2002, stars two of Hong Kong’s biggest film stars. Tony Leung, best known for his role in Wong Kar Wai’s 2000 film, “In the Mood for Love,” which earned him best male actor at the Cannes Film Festival, and Andy Lau, one of Hong Kong’s most popular and prolific actors with more than 130 film roles to his credit.

The crime thriller’s popularity spawned a prequel and sequel in Hong Kong, but received very little press during its short U.S. release in 2004.

In the original, the majority of the film’s climactic scenes take place on bleak high-rise rooftops. The violence is quick and jolting, but never excessive. The romantic sidebars are remarkably downplayed. The colors are out, and the films stars never distract from the film’s tightly woven plot.

In Scorsese’s interpretation, the blood flows freely, a full-blown love triangle breaks out involving the two moles and a police psychologist played by Vera Farmiga, and the characters at times overshadow plot.

In one scene, a paranoid Nicholson, playing crime boss Frank Costello, shatters DiCaprio’s freshly cast broken arm in search of a police wire, and then slams the exposed arm with a boot. DiCaprio writhes in pain and falls to the floor dramatically, clutching his wrist.

In the same scene in Infernal Affairs, Leung’s cast is broken without warning in a moment of seething rage by Eric Tsang, playing Nicholson’s Hong Kong counterpart. Leung bites his lip and grabs his arm, and then the action swiftly moves on.

Scorsese flushes out his characters fully leaving no question about the driving forces behind their actions: guilt, ambition, and paranoia. In contrast, the Hong Kong original leaves most of the inner motivation up for interpretation.

At times overindulgent, “The Departed” turns a slick thriller into an epic crime drama, sacrificing succinct plot for wincing violence and thick character layering. But anyone who enjoys this film needs to look at its slimmer Hong Kong predecessor.