Every few years, former wrestler Dwayne “The Rock” Johnson drops his action-hero scowl and flexes his comedic muscles. In movies like “The Other Guys,” “Pain & Gain” and even “The Tooth Fairy,” he’s consistently great. So why is it so surprising that he steals the show in the new action-comedy “Central Intelligence”?

Maybe it’s because Johnson has never played a character quite like Robbie Weirdicht, an overweight kid who spent high school as a punching bag for bullies. His only defender was the coolest overachiever in school, Calvin Joyner (Kevin Hart), who despite his varsity jacket, student council tenure and well-received “Hamlet” always treated Robbie with kindness. Twenty years later, Robbie has transformed himself into a CIA superagent — now played by the hulking Johnson we all know — but as one character cruelly notes, “Once a fat kid, always a fat kid.”

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Is that a role The Rock can convincingly play? You bet. “Central Intelligence” helps him along by introducing us to the young Robbie (a digitally altered Sione Kelepi) in an opening high school scene that is both funny and heartbreaking. Still, it’s Johnson who makes us believe Robbie is real. Throughout the film, he never misses a beat as he alternates between a pumped-up action hero and a fashion-challenged misfit. “You’re like Jason Bourne,” Calvin marvels, “but in jorts!”

Hart, too, is dependably funny and jittery as Calvin. He’s the classic average Joe who’s pulled into a wild plot involving an arms dealer, an encrypted database and a steely CIA honcho (Amy Ryan) trying to take Johnson down as a rogue agent. As always, Hart also gives his character a heart: Calvin’s promising youth has devolved into workaday adulthood, so maybe a little adventure is exactly what he needs.

This is standard stuff, but director and co-writer Rawson Marshall Thurber (“Dodgeball”) proves handy with action sequences (Robbie’s spectacular decimation of Calvin’s office is a treat) as well as classic shtick (Johnson even gets to don a goofy disguise), and there are moments of real camaraderie between the two stars.

“Central Intelligence” may be a buddy-comedy, but it’s Johnson’s movie all the way.