Earl Brooks is not your average serial killer, and that’s precisely what makes him such an entertaining figure to scrutinize. He’s the head of a successful corporation, with a wife at home and a daughter away at college. We meet him as he’s being honored as man of the year and we watch as he humbly attends Alcoholics Anonymous meetings in hopes of facing and curing his addiction. He’s a man to look up to, a man who’s earned the dignity of being addressed as “Mr.”

But unlike a killer such as Hanibal Lecter, who always relished his evil nature, Mr. Brooks (Kevin Costner) is appalled by what he’s frequently compelled to do. He desperately wants to stop, so much so that he has disassociated the part of himself that enjoys heading out into the night, methodically breaking into people’s apartments, putting a bullet or two through their heads, and arranging their bodies — typically those of two lovers — in sexually suggestive (or is it romantic?) poses.

See, while it’s Mr. Brooks who murders these people, it’s not his desire. Rather, his gruesome acts are the wishes of a balding man named Marshall (William Hurt), who seduces and convinces Mr. Brooks to do these things. More than just a split personality, Mr. Brooks has manifested Marshall as a complete person, someone who is tangibly there in the car and at the murder scene, egging him on.

It’s an interesting style for a thriller to use, something of a literal variation on what occurs only psychologically in “Dr. Jekyll and Mr. Hyde.” It makes possible a series of riveting discussions that give us rare insight into the mind of this murderer, in the form of a debate between the deranged figment of Earl’s imagination and the man struggling to take the moral path. As Mr. Brooks tries to put down the knife for good, it’s Marshall who laughs and says this abstinence won’t last. When Mr. Brooks leaves the curtains open one night as he murders a couple, it’s Marshall whom Brooks chides for allowing him to get sloppy and make a mistake that might cost him and his family all the respectability he has fought to build.

In the film’s most intriguing scene, Mr. Brooks and his alter ego systematically debate the intentions of a man whom they come to call Mr. Smith (Dane Cook), who sends Mr. Brooks a package with photographs of that murder scene, shot through the open curtains. Aloud, Mr. Brooks and Marshall ponder: Does he have copies? Why didn’t he just go to the police? Would it be easier just to murder him, right here in the company’s conference room, then to let him walk out the door and pose a threat to everything that Mr. Brooks has built for himself?

But Mr. Smith is not a money hound, and no proper citizen. He doesn’t want to blackmail Mr. Brooks or turn him in to the police to help protect society. He has quite a different agenda that requires Mr. Brooks’s assistance. And given the fact that Detective Tracy Atwood (Demi Moore) has already conducted a preliminary interview with Mr. Smith in hopes of breaking the case, Mr. Brooks is inclined to do just about anything that he asks, providing Mr. Smith keeps his mouth shut.

Featuring an unlikely cast, including two of Hollywood’s most faded stars and a wild card in the form of Mr. Cook, who has a successful stand-up comedy career, “Mr. Brooks” might look like a movie to avoid. But partly out of skill, and partly out of luck, this is a thriller that moves at a brisk clip and sustains a considerable amount of tension. Playing Mr. Brooks as a sitff chameleon, Mr. Costner balances the heart of a family man with the shrewdness of a businessman and the bloodlust of a murderer. As he dons black and assembles his silencer on his way out to the next kill, Mr. Costner projects an aura of cold, ruthless detachment that will have his “Field of Dreams” fans reconsidering just what this man of the year is capable of.

Given her somewhat throwaway part, Ms. Moore cuts to the heart of Atwood’s desperation — that while a divorce has thrown her personal life into a cloud of shame, her professional life has offered a last chance to prove her worth. And Mr. Cook, asked basically to play the crazy voyeur, plays crazy with verve.

While the movie occasionally reaches for extraneous subplots to add to the stakes, director and cowriter Bruce A. Evans almost haphazardly hits gold in a final subplot that involves Mr. Brooks’s family, and the possibility that his defects have been passed down.

As all these threads begin to wrap around one another, “Mr. Brooks” emerges as the most entertaining entry of the summer thus far — a horror film with an intriguing villain, a mystery that engages us with an intellectual debate between a murderer and his own subconscious, and a moral tale that confronts us with the question of what we would do if we were the monster.