Everyone in Mexico knows the scene. Darkly handsome in a dinner jacket, actor John Gavin stands in an elegant wood-paneled billiard room, glass in hand. "Have you," he asks in Spanish, "tried the test of maturity?" For more than two years Gavin's suave salesmanship for Bacardi rum has been a staple on Mexico's commercial television. To most viewers, Gavin seemed unremarkable: just a middle-aged American actor earning a few pesos as a video pitchman. That benign image changed when Ronald Reagan announced that he had picked his 51-year-old Hollywood pal for the sensitive post of U.S. ambassador to Mexico. Suddenly the rum commercial became a symbol of America's trifling with its neighbor—and a sore point as Mexicans wondered whether Gavin, or any actor, could pass the diplomatic test of maturity. "Our country deserves greater respect and better treatment," wrote a columnist in the nation's largest newspaper, La Prensa. Another commentator said acerbically that if Mexico had to be saddled with a performer, "personally, I would prefer Wonder Woman."

The Gavin controversy is more serious than these snappy one-liners imply. Before the appointment was officially announced, a leading Mexican newspaper printed a distorted account of a speech Gavin delivered in Los Angeles last summer. In it he criticized Mexico's system of land reform—an emotional cause in a country where one million died between 1910 and 1917 in a revolution to restore land to the peasants who worked it. The state governor of Querétaro dismissed Gavin's remarks as "foolishness." The country's largest peasant organization expressed "profound concern that a man with that mentality could become the United States envoy to Mexico." Faustino Alba Zavala, acting head of Mexico's official labor coalition, charged that Gavin's "erroneous" remarks "showed little tact and demonstrated hardly any diplomacy." But later reports indicated that Gavin's criticisms had been exaggerated in translation.

A former U.S. ambassador to Mexico, Joseph Jova, insists Gavin is a sound choice: "He has studied economics and Latin American issues. He's fluent in Spanish, good-looking, knows his way around and is a friend of President Reagan." Indeed, the Stanford-educated Gavin, who holds a bachelor's degree in history, is the offspring of a Mexican mother and an American father, and learned his impeccable Spanish as a toddler growing up in Los Angeles.

After the first reports of his appointment, Gavin quit the Broadway-bound revival of Can-Can and went into seclusion at his Palm Springs, Calif, home. He has declined comment on the controversy until this week's Senate confirmation hearings.

Ironically, John Gavin dreamed of a Foreign Service career long before he drifted, almost accidentally, into acting. After his college graduation he served as a Pan American Affairs officer in the Navy. On leaving the service in 1955, he was headed for the diplomatic corps but was waylaid into acting by a friend's casual suggestion. He has been in the business ever since, starring in film classics like Psycho and Spartacus as well as clunkers like Tammy, Tell Me True and Pussycat, Pussycat, I Love You. Gavin appeared in the TV series Destry and Convoy in the mid-'60s and appeared on Broadway in the musical Seesaw in 1973. Lately his work has consisted largely of cameos in Love Boat and Fantasy Island.

Never completely captivated by acting, however, Gavin always tried to keep at least one foot in striped pants. Through the '60s he served as special adviser to José Mora and Galo Plaza, secretaries general of the Organization of American States. In 1971 Gavin was elected president of the Screen Actors' Guild, the position that launched Ronald Reagan on his political career. "If he doesn't make it as an actor," TV Guide once suggested, "TV's Destry may yet become a diplomat."

Now John Gavin's long-deferred career will commence very quickly: The White House wants him on the job when President Reagan makes his next foreign trip—to Mexico. Meanwhile the Bacardi commercial has been pulled off the air. "We felt it wouldn't be honorable," said ad executive Alejandro Cuevas, "to project the commercial's image of the man."