Isabella Rossellini puts down her fork, straightens her back and shows me how she captured a pivotal moment in her new film, “Conclave,” a thriller set in the Vatican and set in a world far removed from the 70-acre Long Island farm where we’re having lunch. In the scene, Rossellini’s character, a nun named Sister Agnes, is navigating a dark hallway, trying to remain unnoticed, when she sees something mysterious happening a few feet ahead of her.

“My heart has to beat much faster, so the camera picks it up,” Rossellini says. “Your breathing has to translate that. So I…” And with that she inhales, as quietly as she can, filling the room with a sense of tension without saying anything.

It turns out that good acting on screen is all about waiting to breathe.

In “Conclave,” Sister Agnes tends to the brotherhood of cardinals gathered to elect the next pope. She’s often seen wandering the perimeter of the male-dominated deliberations, but she’s rarely heard. That required Rossellini to convey reservoirs of distrust with a mere look or gesture. Yet with just a few lines, she nearly steals the film from heavyweights Ralph Fiennes, John Lithgow and Stanley Tucci, who banter, bluff and debate as they outsmart each other to gain control of the Catholic Church.