Did former presidential candidate Rudy Giuliani influence Meryl Streep’s portrayal of conservative avenging nun Sister Aloysius in “Doubt”? Consider, there is that moral certitude, the strange pursing of the lips, the occasionally maniacal righteousness. “I’ll never tell,” giggles Streep, then she slyly suggests a certain affinity between the two, that like Sister Aloysius, who’s quite convinced of her own rectitude, Giuliani “is one of these people who is sure. He’s sure. It’s a great quality in a leader. People don’t like vacillation. You know, if you trust your life or your children to this person, you want a hard… there in charge at St. Nicholas,” the Bronx school where “Doubt” is set. Well, yes, maybe you did in 1964, when the movie takes place. The film, which John Patrick Shanley directed and adapted from his Pulitzer Prize-winning play, is a dialogue-driven showdown between traditional disciplinarian Sister Aloysius, the school’s principal, and liberal, progressive priest Father Flynn ( Philip Seymour Hoffman), whom she suspects of having molested the school’s only black student. The evidence is inconclusive, but Sister Aloysius’ certainty is unshakable. Unlike the stern, humorless Sister Aloysius, who could very well give anxiety attacks to anyone who attended such Catholic schools in the mid-’60s, la grande Streep is loose and wry in person. That’s somewhat an achievement in Zen, considering she’s spending much of this Oscar season dutifully visiting the various guilds in Los Angeles, giving talks and interviews about the film. The 59-year-old is dressed in an oversized glossy olive shirt, jeans and square glasses that slightly obscure her luminous skin and high cheekbones (which appear ridiculously undiminished by time). Streep has earned the most Oscar nominations of any actor (she’s now at 15 with last week’s lead actress nod for “Doubt”), though she has won only two, the last more than 25 years ago for “Sophie’s Choice.” There have been myriad trips to the Academy Awards with nothing to take home but memories. None of which has eased Streep’s perennial discomfort with the red-carpet frenzy. “I don’t know how to be,” she says. “I just always feel like I’m in costume. And I am. And borrowed jewelry. The event is weird. The thing of feeling part of the community, the acting community and the film community… that’s kind of great. To see people whose work I really do admire and I really don’t know them, so I get to come up like a fan and say, ‘I really thought you were great in this.’ That’s fun. But not the hyperbole of the whole… ” she struggles for a word. Hoopla? “It gets insane. Have you ever walked the red carpet?”

Actually, no.

“Your head explodes,” she says, describing the experience. “And [the reporters are] shouting at you, ‘What do you think? Are you excited?’ Every year, I try to figure out how I can go to the Oscars and not do the red carpet.” Once, her “Ironweed” and “Heartburn” costar Jack Nicholson “told me where the back door at the Shrine was and I couldn’t find it,” she said of the former home to the Academy Awards. When Shanley was putting together the film, it was abundantly clear to him who should play Sister Aloysius. “If I asked 10 people on the subway who should play the part, they’d all say Meryl Streep. It was a no-brainer,” Shanley says. “A role and actress pull together like two planets. She’s it.” Streep enfolded herself into every aspect of the stern nun’s personality. Or, as Shanley describes her process, “If I’d asked her how to drive a sports car as Sister Aloysius, she’d know how Sister Aloysius would have done that.”

Streep prepared by spending time with Sister Peggy (Shanley’s real first-grade teacher) and other nuns. Her efforts were rewarded Sunday night with a SAG award for lead actress. “Nuns are easy comedy, so we have made fun of them from Monty Python to Chris Durang, but I think there’s something that’s confounding to the outside world about women who reject all the things that most women build their entire lives around, which is getting a man, getting a husband and the children and looking good,” Streep says. “And they just jettison all of that, and there’s great liberation in that, I think.” It’s long been Streep’s inclination to inhabit women who’ve somehow been misjudged, even marginalized, by society and give them back their dignity. Such roles include Holocaust survivor Sophie Zawistowski in “Sophie’s Choice,” Lindy Chamberlain, who lost her baby to a dingo in “A Cry in the Dark,” and even Miranda Priestly, the icy fashion magazine editor that Streep plays so effectively in the comedy “The Devil Wears Prada.” What gives the fashionista Priestly her zing is not just the hauteur, but also her unexpected pathos.

The actress clearly also takes a sympathetic view of Sister Aloysius as someone who is driven to fight against the misogyny of the church, to protect the innocent no matter the cost. But is she right? With its purposeful ambiguity, “Doubt” is designed to divide audiences and leave them battling over whether Father Flynn is guilty. Striding down the school halls, her ferocity and anger barely contained by her black habit, Sister Aloysius can be seen either as a retrograde, pleasure-denying authoritarian or as a righteous angel of the defenseless. Streep says she thought of Sister Aloysius like the Jack Nicholson character in “A Few Good Men,” the obstreperous Marine colonel who famously yells, “You can’t handle the truth!” when his tactics are questioned. Female truth-tellers have always had a hard time, particularly those like Sister Aloysius.

“She’s a dragon and scary and hideous to us, I think, because she’s a woman,” Streep says with great certainty, “but she’s just doing Jack’s job.”