A movie about a man with a split personality is interesting. A movie that has a split personality itself is not, unless both sides are equally appealing. In Mr. Brooks, they are not.

An addiction to killing must be an awful burden to bear. You want to stop, and know you should stop, but can’t. One of the smartest things Mr. Brooks does is take us inside the mind of a serial killer, allowing us to see and hear how he meticulously plans every detail so he doesn’t get caught while simultaneously fighting his temptation to commit the murder at all. Eventually you’d think he’d sabotage himself, but director Bruce A. Evans’ film is smarter than that.

If the entire movie were about the devious thoughts of the killer, this would be something wholly original and special. But for stupid reasons the subplot takes the story in drastically unnecessary directions, and ruins what is otherwise a well-told psychological thriller.

Earl Brooks (Kevin Costner) is a very successful businessman and a true icon in the Portland, Ore., community. In fact, as the movie opens he’s named Chamber of Commerce Man of the Year. He has a beautiful wife (Marg Helgenberger, CSI) and daughter (Danielle Panabaker), a lovely home and more money than he knows what to do with.

He also carries a distinction he’s not quite as proud of: He’s the infamous “thumbprint killer” whom detective Tracy Atwood (Demi Moore) has been tracking for years, but has never come close to capturing. Part of the reason for this is the brilliant and cunning help of Marshall (William Hurt), Mr. Brooks’ alter ego whom only he can see and hear. Marshall is the Hyde to Brooks’ Dr. Jekyll, the devil on his shoulder who relishes in convincing him that the wrong thing to do is what’s right for them. This could’ve come across as a cheap, silly trick, but Costner and Hurt have such great chemistry that together they’re the most interesting character the movies have seen in a long time.

Mr. Brooks’ storyline also includes a young photographer called Mr. Smith (Dane Cook), who has a photo of him committing his last murder and is blackmailing Brooks into teaching him how to kill. This, along with the subplot of why Brooks’ daughter has mysteriously returned home from college, has the makings of a great psychological thriller.

But co-writers Evans and Raynold Gideon also felt the need to flesh out the role of the detective, and it is here that they go painfully awry. So much time is taken with Tracy’s punk ex-husband (Jason Lewis), his shyster lawyer (Reiko Aylesworth) and their divorce that it’s too big a diversion from the more interesting main story. When big-name stars such as Costner and Moore don’t share one scene together, you can’t help but wonder why they’re in the same movie.

More important, Tracy’s bitter divorce battle is egregiously generic and something we’ve seen countless times before, as is the character arc of a wayward detective with personal problems trying to track the killer. There’s nothing original, notable or interesting about this, and it nearly destroys what could’ve been a great movie.

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Mr. Brooks **1/2   Directed by Bruce A. Evans. Written by Evans and Raynold Gideon. Starring Kevin Costner, Demi Moore, Dane Cook, William Hurt, Marg Helgenberger. Rated R.   **** A genuine must-see ***  Entertaining **   Mediocre but not worthless *    A wretched waste of time   Also opening in Miami-Dade County this Friday: Gracie, Knocked Up.