President Donald Trump’s sudden announcement on Sunday that he intends to impose a “100 percent tariff” on movies produced abroad jolted advocates who have been working for months to return more film and television production to California.

Between the Stay in LA coalition, the Hollywood guilds, the state legislature and Gov. Gavin Newsom, production insiders thought they knew the players and what their positions were. But, as the White House and Trump ambassador Jon Voight took a stab at offering up a plan, studio chiefs are mostly staying silent (publicly) while union leaders’ considered statements have signaled guilds are keeping their options open.

There’s general skepticism that the tariff will actually be implemented and raised eyebrows about Trump’s motives, as many of these industry types express caution and say they’re waiting for details on the policy. A group of film studio executives are expected to speak with...