“Alex Cross” is an interesting exercise in back-engineering, a prequel that takes us back to the days before the psychologist/police profiler was the sage, solemn and inscrutable sleuth Morgan Freeman ably brought to the screen in two films over a decade ago.

This Cross is cocky, a bit trigger-happy, prone to revenge, a real “action hero.” And this Cross is played by Tyler Perry.

But by definition, he’s less interesting. When you fill in somebody’s back story, you strip away their “loner” mystique. When you focus on the flippant in a film about a frantic hunt for a psychopathic assassin, you diminish the urgency of the hunt and remove the gravitas of the character.

And when you make Tyler Perry run and point a gun, you remember why nobody’s ever used him as an action figure before.

We meet Cross as a domesticated and revered Detroit “detective-doctor,” a hyper-observant wizard his colleagues (Edward Burns, Rachel Nichols) call “Gandalf,” a man his boss (John C. McGinley) can point at a crime and say, “Solve it, please.”

That’s what happens when an unnamed killer tortures and murders a rich woman with a penchant for mixed martial arts fighters. Matthew Fox is a coiled spring of tension in this part — lean, all muscles and tattoos and shaved head. He has the budget, the gadgets and the mania for assaulting members of a company involved in Detroit redevelopment, no matter what security measures they take.

He’s also something of a psychotic cliche — twitchy, with blurry flashbacks that make him snap just as he’s about to remove somebody’s fingers or shoot out their eyes. He does Picasso-tribute charcoal sketches that he leaves at the crime scenes — “clues” — and drives a charcoal-colored Cadillac CTS, a chunk of product placement so blatant (among other General Motors plugs) as to deserve its own billing.

Cross will cross swords and wits with the killer, who calls him to taunt him. “Confucius said, ‘When setting off on the path of revenge, dig two graves.”

Will Cross get his man, and will he pay the price?

The script is freely adapted from James Patterson’s “origin story” novel and is packed with indulgent dumbing down. German security folk snap “You EEEdiots” at the Detroit cops, characters say unnaturally flattering things about other characters (Jean Reno is a tycoon under threat). Much of the movie is Cross’s home life — happily married (Carmen Ejogo), father of two, who keeps his “Nana Mama” (feisty Cicely Tyson) in the house with him, as cook and dispenser of the wisdom of the ages.

Rob (“xXx”) Cohen pays more attention to the shootouts and fights than the flow of the film, never fretting that there isn’t a moment’s suspense, never letting us feel for the victims.

“Alex Cross” is not an awful movie, but it isn’t a very compelling one. Cohen, the screenwriters and Perry share the blame for that.

If “Kiss the Girls” and “Along Came a Spider” had been this weak, there’d have been no reason, no urge to revisit the sad, serious character Freeman brought to life so vividly.