A weird thing happened to Mariana di Girólamo while she was working on Wild Horse Nine . The Chilean actress, a native Spanish speaker who had never been on an American production before, started having vivid dreams — in English.

Di Girólamo labored over her preparation for the role, first perfecting her sides for her audition self-tape and, later, after landing the part, working through the script with both her acting coach and director Martin McDonagh to master the distinctive rhythm of his dialogue. For a native speaker of Chilean Spanish, English required some adjustment — especially the idiosyncratic English in McDonagh’s screenplay about two CIA agents dispatched to a remote Chilean island on the eve of the 1973 coup. “We had to go over every comma,” she jokes.

When she started dreaming in English, she knew something had clicked.…