James Bond. An astronaut. A Secret Service agent.

Hollywood's most famous actors have played these characters in big-budget movies recently, and a new social media project wants to know: Why haven't Hollywood's traditional leading roles been offered to more Asians?

The project, called #StarringJohnCho[1], is the latest high-profile attempt by activists to call for greater diversity in entertainment, but it may be the first to use one actor as a meme to break down the archetype of a Hollywood leading man.

William Yu, the 25-year-old mind behind the project, modified posters of well-known movies to show what the Korean-American actor John Cho would look like if he had starred in, say, "The Martian," "Spectre," and "London Has Fallen." All have white male leads.

Mr. Yu, a digital strategist who works in New York City, created the website on Thursday. Since then, the hashtag, #StarringJohnCho[2], has been shared thousands of times, spurred by coverage from Mic[3] and the Angry Asian Man[4] blog.

Mr. Yu said that he had been motivated after Asian-Americans were left out of #OscarsSoWhite, another campaign against whitewashing in Hollywood.

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"It was very much a two-sided argument," he said in an interview. "You had a white issue, and you had an African-American issue."

Recent high-profile casting decisions, including the Scarlett Johansson role in the remake of the Japanese anime film "Ghost in the Shell" and Tilda Swinton's casting[7] as a sorcerer of Tibetan descent in Marvel's "Dr. Strange," also served as inspiration. But Mr. Yu said the final push came when he read a recent study by researchers at the Ralph J. Bunche Center for African American Studies at the University of California, Los Angeles that suggested movies with more diverse casts made more money[8].

"That was kind of the linchpin of me thinking, 'If that's true, then why aren't we seeing leads reflecting this fact?' " he said. "If they're not casting these leads, let's show what Hollywood would look like if they did."

Mr. Yu said he didn't grow up with many Asian role models in movies or on TV, and he didn't have many mainstream actors to choose from when creating the project. As he Photoshopped Mr. Cho into the role of Captain America, he caught himself chuckling at the possibility that an Asian man could fill the role.

"It shouldn't be crazy for someone to take that role," he said.

Mr. Cho, 43, hasn't addressed the project — save for sending a heart emoji[9] to the @StarringJohnCho Twitter account. But he spoke publicly about how racism had affected his professional life in a session[10] with Reddit users last year.

"I experienced racism, and in my professional life, I try to take roles (and have always tried to take roles) that don't fall within the parameters of any Asian stereotype," he wrote.