"Some days the first coffee just laughs at you," Katherine Waterston is saying wearily to the cup in front of her. "It says, 'Oh, you think I'm going to wake you up? Sucker.'" If Waterston is having a hard time rising and shining, there's no evidence of it on her apple-cheeked face, framed charmingly with frizz protruding from a careless bun. We've settled in at New York's the Smile, in Noho, not far from her apartment, and in her white T-shirt and jeans, with a silver dog tag hanging from her neck, Waterston looks like a tomboyish NYU student, not a 35-year-old actress who has been at it for nearly 10 years. "My agents call me Benjamin Button," she admits sheepishly.

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Last year Waterston blew critics away with her turn as Joaquin Phoenix's languorous and troubled hippie girlfriend in Paul Thomas Anderson's adaptation of Thomas Pynchon's Inherent Vice. This month she's starring as Chrisann Brennan, the longtime girlfriend of Apple's founder (played by Michael Fassbender) and the mother of his first child, in Steve Jobs, directed by Danny Boyle and written by Aaron Sorkin. With just those two roles—parts that would generally have gone to women 10 years her junior (think Emma Stone or Jennifer Lawrence)—Waterston became a Hollywood rarity: an actress who is cast opposite age-appropriate men. "It's not like these studios are casting me because they want a woman who looks 35," Waterston stresses quickly. "It's just that, as you said, I look like a child!"

"Oh, god," she adds. "Other actors are going to read this and think, What an asshole."

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Waterston's good fortune, even if genetically bestowed, is well deserved. She spent her actual twenties essentially being ignored by Hollywood, even as she raked in glowing notices for theater performances and the occasional small or independent film role. Vice changed all that. Suddenly producers and directors all over town were enthralled with their "new" discovery. "Katherine has this tremendous power and poise that translates on film into searing honesty," says Boyle. "Some actors are interested in the technicalities, like where the camera is. She's not. She doesn't do anything for effect. She brings a wonderful lack of guile."

New York theater insiders, and no shortage of casting directors, have known this about Waterston for years, which made it all the more frustrating when she wasn't getting parts. "There was 10 years of obscurity prior to this moment," she says. "I thought, I know how this works. I don't stand a shot when I'm in my thirties."

Adam Rapp, the Pulitzer Prize–nominated playwright and director who has cast Waterston in many of his plays and was her real-life romantic partner for several years, has a theory. "She's six feet tall," he says. "And when you're filming with a scene partner who is five-foot-nine, you have to put him on an apple box. Even in theater you're changing the horizon line with her, so by casting her you are making a really strong choice. She's very good, and over the years came close to getting a lot of jobs but ultimately lost out to big stars. A lot of it was that they'd have been taking a risk casting such an Amazonian beauty."

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Had P.T. Anderson not taken the risk, Waterston might be out of the acting game altogether. "I think I really would have quit if I hadn't gotten Inherent Vice," she says. "Or maybe I would have just shriveled up and died."

More likely she would have taken it in stride. Waterston comes from a family in which the ups and downs of the entertainment industry are taken for granted, and performance is done for performance's sake alone. Her father is Sam Waterston, the stage and screen stalwart who has starred in a slew of Woody Allen movies, Law & Order, and now Netflix's Grace and Frankie, with Lily Tomlin and Jane Fonda. Her mother Lynn Louisa Woodruff was a top fashion model and, Katherine says, a natural performer and storyteller. "The assumption is that my father is the influence, but they both were, as well as my uncle, a painter who's one of the best stand-up comics I've ever seen—but just in my kitchen." Her older half-brother, James Waterston, and her older sister, Elisabeth, are also actors, and younger brother Graham is a filmmaker.

Waterston's Connecticut childhood was happy and typical enough, save maybe for an age-inappropriate film diet. "I was into some heavy stuff on VHS: In the Name of the Father, Breaking the Waves," she says. "We had a strange collection of movies at home. We didn't have a grocery store or a stoplight in my tiny hometown, but we had a great video store, and the owner, Rollin, would recommend movies like Spanking the Monkey to us when we were kids. I got my education from Rollin and the awards screeners we got from the studios." (It wasn't all unwholesome fare; she still maintains an obsession with The Dick Cavett Show. "It is, hands down, my favorite thing that has ever been on TV. I've seen every episode.")

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Professional aspirations didn't come until later. "Oh, god, no," Waterston says, mildly horrified, when asked if she was a child actor. "I was so naive about the business. Maybe I just perceived it as a grown-up world because it was what my father was doing. I saw him with child actors; I was on set in Louisiana the summer he filmed The Man in the Moon with Reese Witherspoon. I must have been around 10, and I remember feeling some sort of fascination with what Reese was getting to do. But I also felt like it was very separate from me. I didn't know to want to be doing that."

When the subject of high school comes up, Waterston becomes guarded and refuses to name her school, though any interested Googler can learn that her dad was the commencement speaker when she graduated from Loomis Chaffee, a top East Coast boarding school, in 1998. (Sam attended Groton before heading to Yale.) "I am probably overly paranoid about it, but I think knowing these things can make it a little more challenging for an audience to believe you in other worlds," she says. Perhaps, though one could easily argue that any doubts about her being able to transcend her bona fide Mayflower heritage onscreen were probably dispelled when the world saw her goad, via nude monologue, a stoned Phoenix into spanking her repeatedly and enthusiastically in Vice.

But try as she might to keep her private life private, Waterston can't help but gush about her dad. Asked about her favorite of his roles, she pretty much lists everything on his IMDB page. "I am obsessed with him. I think he's such a brilliant actor," she says. He didn't exactly grease the wheels for her, though.

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"People think the advantage of a parent in the business is that they'll open doors for you. But the true advantage for me is having someone who knows exactly what you're going through. The business is mysterious and hard to navigate, and he really feels that way too. So he's been more of a comforter than an advice-giver, because when it comes to advice, he's like, 'Shit, I don't know.'" She thinks for a moment and grins impishly. "Maybe he really could have helped me open doors and just didn't!"

Nowadays it's not so much comfort she requires from her dad as help preparing for roles. "It's fun, because I have someone to talk to who doesn't glaze over after 20 minutes of me going down a rabbit hole about what my character is about. My dad and I, we can really go down it together. And it's one of my favorite things to do with my time."

"Katherine puts work into it in a way that I don't. I don't have a process; I sort of fly by the seat of my pants," says actress Gaby Hoffmann, Waterston's close friend and her co-star in the recent independent film Manhattan Romance. "She works so hard that she reaches this point of total freedom. I'm always amazed that she can put so much thought into it and then arrive at this place where it all seems like it's coming from intuition alone."

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On the afternoon of our meeting, Waterston has plans to buy a new suitcase: Later that week she's off to London for five months to begin filming the first installment of the screen adaptation of Fantastic Beasts and Where to Find Them, J.K. Rowling's crazily anticipated Harry Potter spinoff, which is guaranteed to keep her working—famous, even—for the next decade or so. She reportedly beat out Dakota Fanning and Saoirse Ronan (both 21) for the lead role of the witch, Tina, opposite Eddie Redmayne—this after initial reports had Waterston testing for Tina's older sister, Queenie. Score one more for that cherubic face.

"It still never feels like smooth sailing," Waterston says. "About 10 minutes before I found out that I had landed Fantastic Beasts, I got a residual check in the mail for zero dollars. On the check it said 'Advice Slip.' And I was like, 'Well, what's the advice? Go into another line of work?'"

Then the phone rang.

This article originally appeared in the November 2015 issue of Town & Country.