To the day he dies, Whitey Bulger will deny that he was a rat. A crime boss who ruled segments of Boston for some 20 years, he fled the city in 1994 when it appeared his arrest was imminent. After he was finally captured in 2011, he eventually went on trial and was convicted on 31 counts, including 11 related to murder.

Adapted by Mark Mallouk and Jez Butterworth from a book by veteran journalists Dick Lehr and Gerard O’Neill, Black Mass is fascinating, if not always as rocket-fueled as one might hope. Director Scott Cooper (Crazy Heart, Out of the Furnace) fashions a good drama that has far more lighter moments than one might expect. It’s as though the filmmakers decided that the stuff of real life might come across too strongly, too sternly, and too ugly for most viewers, and Black Mass very much wants to be a mainstream entertainment product.

That’s apparent in the casting of Johnny Depp as Whitey Bulger. The criminal is a shadowy yet dominating figure in the book, but the early part of the movie appears devoted to building a case for him, highlighting his neighborly acts toward members of his community, and showcasing his relationship with Lindsey Cyr (Dakota Johnson), with whom he had a son. Often the accent is on the comic side of things, the better to contrast with Bulger’s ruthless, unflinchingly murderous criminal activity.

Of necessity, the movie juggles the time frame of events, and leaves out key characters. (Bulger’s simultaneous, decades-long relationships with two women are omitted entirely.) But the main story line that emerges is between Bulger and John Connolly (Joel Edgerton), an ambitious FBI agent who recruited Bulger as an informant, only to quickly lose control. Connolly is a difficult character to pin down on screen; his motives are never made clear, beyond his repeated insistence that ‘neighborhood ties are more important than anything,’ or words to that effect. Since he and Bulger both grew up in the rough and tumble neighborhood of South Boston, Connolly speaks often about their supposed kinship, but the much older Bulger never treats him that way.

Cooper asserts his directorial personality by avoiding nearly all things that might remind one of Martin Scorsese, save for an isolated moment or two. That’s quite an achievement in itself, especially since Scorsese marked the Boston crime scene so floridly in The Departed. Instead, Cooper seems to be reaching backward toward something like Peter Yates’ The Friends of Eddie Coyle (1973), which was also set in the Boston criminal underworld but featured a grittier, more streel-level approach.

Black Mass is strengthened by an incredibly deep cast of talented actors, including Benedict Cumberbatch, Kevin Bacon, Peter Sarsgaard, Jesse Plemons, Julianne Nicholson, Corey Stoll, Juno Temple, Rory Cochrane, David Harbour, and W. Earl Brown. They all serve the material well, making the most of their limited screen time to make often-scathing impressions.

And then we come to Mr. Depp, who walks through the movie in a league of his own. He is the star of every moment he is on screen because he is the always the most important character in every scene in which he appears. This is the definition of a star turn by a marquee attraction, playing a role that has been written to flatter the actor. In what should be an ensemble drama, Depp never disappears into his character; he’s always Johnny Depp starring as famed gangster James “Whitey” Bulger in Black Mass, a movie that emulates real life while never quite breathing on its own.

The film opens today in theaters throughout the area.