London — In the papal thriller “Conclave,” Isabella Rossellini’s character, Sister Agnes, is a woman of few words. When she does speak, during a pivotal moment in Edward Berger’s film, it’s from a place of truth, rather than authority or entitlement. But playing a nearly silent woman in a story filled with men was a daunting proposition for Rossellini.

Despite her years of acting experience, she admits to being anxious about stepping onto the set. “We had three or four days of rehearsal, but I was nervous,” says Rossellini, speaking at the Corinthia Hotel after the London Film Festival premiere of “Conclave” in October. “I thought maybe Sister Agnes would be nervous too, so I used that.

I didn’t have to repress it. She’s not part of the brawl with the men. She doesn’t get into the opinion of who should be the next pope.

When she does speak, she speaks what she knows and goes back to her vow of being silent and invisible and obedient.” The film, based on Robert Harris’ 2016 novel, follows Cardinal Thomas Lawrence (Ralph Fiennes) as he shepherds the conclave at the Vatican during which the next pope will be selected. There’s a lot of in-fighting and manipulation, with several cardinals vying for the position, including Stanley Tucci’s Cardinal Bellini, Sergio Castellitto’s Cardinal Tedesco and John Lithgow’s Cardinal Tremblay.

Sister Agnes runs Casa Santa Marta inside the Vatican, where the cardinals are sequestered during the conclave. I.