Until this summer, I was not particularly familiar with John Cena. I knew he was a wrestler, and that he appeared in a lot of Make-A-Wish videos that made me cry at my desk. Then came his small role in “Trainwreck” as Amy Schumer’s character’s boyfriend-slash-someone that autocorrect might change to “duckbuddy.” It’s obvious on the very muscled surface why Cena would be cast, both in “Trainwreck” and in this week’s “Sisters,” alongside Tina Fey and Amy Poehler: Namely, on the surface, he is very muscled.

In Hollywood shorthand, a man this cut is akin to a woman who’s busty and blond: They must be stupid. Cena is not. You know how I know? Because the man can do comedy, and you can’t be funny if you’re dumb.

Comedic acting is tricky business, because to get laughs you have to not try to get laughs. There is nothing more unfunny than someone who laughs at his or her own jokes (this type of mugging is explicitly addressed in “Sisters,” with Bobby Moynihan playing a guy who is always “on” and therefore never funny. If you make a lot of jokes on Facebook and no one ever ‘likes’ them, you are this person.).

In “Trainwreck,” Cena’s effectiveness largely came from the fact that he was playing against type — big guys aren’t supposed to love independent film and cry easily. In “Sisters,” in which Cena plays a drug dealer named Pazuzu, his intimidating look makes more sense, so he can’t rely on visual incongruity to bring the funny. Cena still gets the laughs, though, because he has the rare ability to say ridiculous things with utter sincerity. That’s not something that comes easily, and it takes an immense amount of smarts to pull off.

Cena’s comedic gifts — like those of Dwayne Johnson and Channing Tatum, who are also in that very-low-body-fat boat — prove that to build your muscles you don’t have to shrink your brain.

More columns from Kristen Page-Kirby