A killing machine riding a conveyor belt

★¼55 Rated R. 111 minutes.

It’s possible that Dylan O’Brien may someday grow into the role of Mitch Rapp — the dangerously lethal terrorist-hunter hero of the late Vince Flynn’s popular series of pulpy spy novels — but in the cinematic origin story “American Assassin,” based on one of Flynn’s books, he ain’t there yet.

Despite his scruffy beard and some passable action sequences, the actor, who just turned 26, still comes across as a not terribly interesting, snot-nosed kid: an only slightly older and only mildly angrier version of the adolescent character he played — and continues to play — in the “Maze Runner” movies.

Clearly, CBS Films, which has bought the rights to Flynn’s Mitch Rapp books, is hoping the character has legs. But first, “Assassin” has to make some money. If Flynn’s die-hard fans don’t buy O’Brien’s pretty boy as a killing machine, there may not be a second — let alone a third — opportunity for the actor to grow up.

“American Assassin” opens on the beach in Ibiza, as the still-soft-andsensitive version of Mitch is proposing to his girlfriend (Charlotte Vega). But after she is killed by Islamic terrorists, Mitch undergoes a kind of physical and political transformation that only occurs in Hollywood, or in the pages of potboilers. The movie fast-forwards 18 months to a new and improved version of our hero — who has managed to train himself in martial arts and weapons handling, as well as Arabic language and culture. Mitch 2.0 is preparing to single-handedly infiltrate the radical Islamist militant cell in Libya responsible for the slaughter of the opening scene.

Meanwhile, Mitch is being monitored — and, ultimately, recruited — by the CIA, as its deputy director (Sanaa Lathan) announces that he’s “off the charts” in aptitude for the top-secret Orion program, a specialops commando unit under the tutelage and oversight of ex-Navy SEAL Stan Hurley (Michael Keaton).

The bad news? Mitch doesn’t like to follow rules. The good news? That’s just how we like our action heroes. It’s all as phony as it sounds.

Keaton, at least, looks the part. The story, adapted for the screen by Stephen Schiff, Michael Finch, Edward Zwick and Marshall Herskovitz, and directed by Michael Cuesta (“Kill the Messenger”), leaves most of the toughguy acting to him. When Stan and Mitch go after a rogue Orion graduate who has obtained a nuclear device (Taylor Kitsch), it is Stan, not Mitch, who gets captured and tortured, in scenes that feel like someone behind the camera is taking a little too much pleasure in fingernail removal.

It’s very macho and violent, a la “24,” with women playing secondary and/or disposable characters. Shiva Negar appears briefly, as a mysterious Turkish spy.

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In the end, Mitch Rapp comes across as a poor man’s Jason Bourne. But at least that franchise, after five movies, is only now starting to exhibit signs of battle fatigue. “American Assassin” is just getting started, and it already feels worn out.