The great actress Isabella Rossellini was the face of Lancôme beauty for more than a decade beginning in the early 1980s, but strangely, if you look back, rarely is she the lead in any of her iconic films.

Sometimes a shadow, sometimes on the periphery, a Rossellini character is nonetheless always knowing, from tragic lounge singer Dorothy Vallens in her then-partner David Lynch’s “Blue Velvet” to ethereal beauty whisperer Lisle Von Rhuman in Robert Zemeckis’ “Death Becomes Her” and a wealthy Italian widow who dates below her station in David O. Russell’s “Joy.”

In Edward Berger’s papal potboiler “Conclave,” Rossellini has fewer lines than ever — and probably they could fit on one page — as Sister Agnes. Here is a glowering nun who has seen some shit. In the case of Berger’s English-language follow-up to Oscar-winning heavy-hitter “All Quiet on the Western Front,” Agnes is the eyes...