“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.

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 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 unidentified 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.

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?

“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.