The lobby of the Bowery Hotel was filled with erotic tension. At one end, a young couple made out on a sofa. At the other, a middle-aged woman regaled her companions about a man she might take to bed.  Rooney Mara assessed the situation and made a beeline for her suite to sit for an interview. There, amid a hodgepodge of shopping bags from designer boutiques, a stylist was making red-carpet preparations for the New York Film Festival premiere of “Carol,” Todd Haynes’s sumptuous lesbian romance, spun from the Patricia Highsmith novel “The Price of Salt.”  Public display does not come easily to Ms. Mara, and she distracts herself with an adult version of dress-up: picking out clothes, trying them on and conjuring up a character — one perhaps not quite so much like her sensitive self.  “It’s the only way, really, to get through it,” she said, curling up on a wicker settee on the balcony, a Chanel frock hanging on her bedroom door. “At first I was begrudging about it — I hated being a show pony and a Barbie doll. It’s not normal to go to a movie in a couture dress. And you’re standing there as all these screaming men take your photo.”  In “Carol” — set in 1952, the year the American Psychiatric Association proclaimed homosexuality a “sociopathic personality disturbance” — Carol Aird, played by Cate Blanchett, is a glamorous New Jersey suburbanite whose sexual desires are destroying her marriage and jeopardizing her motherhood.  But the film is just as much Ms. Mara’s: She plays Therese Belivet, a lonely young department-store clerk quietly rebelling against the expectations of her inner circle, particularly the boyfriend who wants to marry her.  When Carol forgets her leather gloves on Therese’s sales counter, a delicate frisson erupts. It’s a maneuver both as thrilling as leaving behind a valuable with someone you hope will call again and as dangerous as accepting candy from a stranger.  The hoped-for call comes, and eventually the women set off cross-country, running from their obligations and into a forbidden love.  At Cannes, where the film had its premiere in May, the response was euphoric. “Almost tout le monde has gone over the moon,” Manohla Dargis wrote in The New York Times, declaring the film “exquisitely directed and acted.” Ms. Mara surprised even herself by winning the best actress award (which she shared with Emmanuelle Bercot of “Mon Roi”).  Ms. Mara, 30, was relaxed and playfully candid on this bright October day, a sylph in black skinny jeans and an outsize sweater with a stripe the same color as the kale salad she nibbled at. But she prickled slightly at the rumor that after the film opens Nov. 20, the Weinstein Company planned to promote her for a best supporting actress Oscar nomination and Ms. Blanchett for best actress.  “I don’t think it’s decided,” she said. (A publicist later confirmed that it had been.)  But both actresses in the same category — would that even be possible?  “It would be possible if we were man and woman,” she replied coolly.  In 1950s New York, two women, one older and one younger, fall in love.  The way Ms. Mara sees it, “Carol” is two films: a coming-of-age tale, as both women grow into the next phase of their lives in ways that will allow them to be true to themselves, cushioned in a love story for the ages.  Mr. Haynes took that story, which the novel anchors in Therese’s point of view, and gave voice to each woman.  In great love stories on film, like “Brief Encounter,” he said, “you’re always on the side of the more vulnerable party.” He added, “And that changes in ‘Carol,’ so by the time we come back to the scene at the very end, we understand the story through both characters’ perspectives and we now find ourselves in Carol’s realm.”  Ms. Mara almost let “Carol” slip away. Originally offered the role on the heels of “The Girl With the Dragon Tattoo,” she felt too depleted after a punishing seven-month shoot in Sweden and too raw from being thrust, virtually unprepared, into the media maelstrom.  “All of the sudden you’re a diva or you’re difficult or you’re a bitch,” she said, almost in apology for a reputation that spread early in her career.  “I passed on ‘Carol,’ which when I think about it now is insane to me, because Cate is one of my favorite actresses in the world,” Ms. Mara said. “Working with her is like a dream come true, so the fact that I passed up on that opportunity — I can only imagine the state of mind I must have been in.”  David Fincher, her director on “Dragon Tattoo,” recalled the fervor surrounding the film as particularly difficult for Ms. Mara. “You have this unknown, and now you put her on every single talk show,” he said. “I think it was surprising to her, and certainly surprising to me, how much people wanted to vivisect her, the person, before she ever showed the goods.”  After completing four more films (Steven Soderbergh’s “Side Effects,” Spike Jonze’s “Her,” David Lowery’s “Ain’t Them Bodies Saints” and a still-untitled Terrence Malick project), then taking a year off to recover, she again received the “Carol” script, by Phyllis Nagy, this time with Mr. Haynes attached.  Her answer was an immediate yes.  “The script and Cate would have been enough, but Todd is brilliant, and I wanted to work with him terribly,” she said. “For me the most important thing is the director. And if the director doesn’t feel like someone I will follow to the ends of the earth, then I just really don’t want to do it.”  Mr. Haynes discovered in Ms. Mara a true collaborator, who, like Ms. Blanchett, brought a seriousness to her role and a meticulousness to her preparation, going so far as to consult a dialect coach to find the precise placement of Therese’s voice in her body — “things that had a directly physical influence on the development of the character,” he said. “This may be all the more impressive given Rooney’s age. She comes to set with a great deal of work and thinking already done on the role and the story.”  Cincinnati stood in for vintage New York, and before shooting began, Mr. Haynes sent her a reference book filled with early 1950s photographs by Ruth Orkin, Esther Bubley and Vivian Maier. (In Ms. Highsmith’s novel, Therese is an aspiring stage designer; Mr. Haynes and Ms. Nagy imagined her as a photographer.)  As for sharing the screen with Ms. Blanchett, Ms. Mara said: “Obviously, playing a character who is just in awe of her — that part was really easy. To just be watching everything she does and figuring out how you can be that way was pretty much what I was doing anyway.”  In an email, Ms. Blanchett returned the compliment. “Rooney plays her acting cards pretty close to her chest, but when she plays her hand, it is breathtaking, beautifully judged, connected and felt,” she wrote.  And getting naked with her formidable co-star?  “Sex is sex, male or female,” Ms. Mara said. “It’s all the same to me.” Perhaps more challenging were the desirous looks filmed almost immediately after the two actresses met.  “We would be going along, and no one would yell, ‘Cut!,’ and we’d be looking at the camera” — she struck a pose of intense longing — “and we wouldn’t know what to do with ourselves,” she recalled. “We were just kind of giggling the whole time.”  Then there were Sandy Powell’s costumes: for Carol, magnificent furs and form-fitting sheath dresses; for Therese, with her limited income, wool skirts and plaid scarves, still beautiful but more obviously worn.  Eventually, Therese’s style comes to echo Carol’s, sleek and refined and accessorized perfectly. “I feel like that’s how most people come of age,” Ms. Mara said. “You learn what kind of woman you want to be by looking at other females.”  Fashion holds a central role in Ms. Mara’s life off-screen as well, and her own quest for identity is peppered with sartorial markers:  • The church clothes she wore as a girl in Bedford, N.Y., to the Sunday football games with her N.F.L. team-founding family — the New York Giants on her father’s side, the Pittsburgh Steelers on her mother’s.  “I was like, this isn’t normal.”  • The tight jeans and tank tops she invested in for auditions at the start of her career.  “I hated having to go in dressed like a slut to get a part for a job I didn’t really want anyway.”  • Even the punk uniform, hacked hair, bleached eyebrows and multiple piercings central to “Dragon Tattoo.”  “Being beautiful didn’t matter anymore, and I felt comfortable for the first time, because I had stopped trying to be someone else’s idea of what I should look like.”  But being beautiful was important as she promoted “Carol” at the New York Film Festival. The Chanel dress awaited her.  “It’s a work of art,” she said, tracing the hem of its heavy two-tiered skirt and the beading on its latticework bodice.  It also, backlit by the midday sun, seemed to portend overexposure.  What would she be wearing under it?  Ms. Mara leveled her gaze. “Nothing.”  A version of this article appears in print on November 1, 2015, on page AR22 of the New York edition with the headline: Wearing a Bold Part Well. Order Reprints| Today's Paper|Subscribe
Ms. Mara embraces her role opposite Cate Blanchett in Todd Haynes’s adaptation of Patricia Highsmith’s novel of lesbian romance.