Why should I trust you?” Dominic Cooper said to Samara Bay, his dialect coach. “Trustya,” she replied, crushing the words together. “Why should I trustya,” Cooper repeated. The actor and coach were standing in the driveway of an old stone mansion in New Orleans’s Garden District, on break from shooting a scene for the AMC series “Preacher.” Cooper, who was born and raised in London, plays the show’s title character, a West Texas preacher possessed by the offspring of an angel and a demon. He tried another line, moving his mouth around the hard twang of the “am” in “vampire,” when the vampire in question — his co-star Joseph Gilgun — interrupted their work.