Like you, Felicity Jones is obsessed with Kate McKinnon's sprightly impersonation of Ruth Bader Ginsburg on "Saturday Night Live."

"She's phenomenal," raves the British actress, who plays a far more grounded but no less fiery version of the Supreme Court justice in the biopic "On the Basis of Sex" (in theaters Christmas Day). "She just totally nails it."

But unlike McKinnon, whose RBG is a trash-talking, breakdancing whiz, Jones has the unenviable task of convincing moviegoers that she's Ginsburg as a clear-eyed young lawyer, whose first gender discrimination case in the early 1970s is depicted in the film.

While Ginsburg, now 85, is a Jewish Brooklynite with a distinct New York accent, Jones, 35, is neither American nor Jewish, unlike original "Basis" star Natalie Portman, who dropped out of the project last year.

But their origins were beside the point for director Mimi Leder, who "kept looking at pictures of young Ruth and comparing them to Felicity," she says. "There was a striking resemblance. I was a huge admirer of Felicity's (movies) and just had this hunch that she could do it."

Jones acknowledges that she wasn't quite as sure: "It was certainly intimidating to take on someone who so many people look up to," including her own mother, who introduced her to Ginsburg’s work years ago. But ultimately, "it was just about taking on every aspect of Ruth and doing justice to the justice."

For starters, there was the accent, which the one-time Oscar nominee (“The Theory of Everything”) worked tirelessly at with a dialect coach, studying early tapes of Ginsburg’s famous Supreme Court dissents for hours.

Walking to and from set while shooting in Montreal last fall, "that was my time to just listen to those in my earphones, and also walk as Ruth and think her thoughts. I became completely obsessive," Jones says. "It was very much about finding the nuances of when you're in a public or private space. So much of Ruth's battle was functioning in a very patriarchal world at that time and how that affected every level of her presentation: not only her voice, but body (language) as well."

As well as changing her voice, Jones wore caps on her teeth and gray contacts to look more like Ginsburg as a young woman.

Given Ginsburg's close involvement to the project (her nephew, Daniel Stiepleman, is the screenwriter), Jones also had access to some of the family’s old home movies, including some taken during her honeymoon with husband Marty (played by Armie Hammer). She even got to spend time with the justice over dinner before shooting.

“I was just (observing) the delicacy and time she takes with everything,” Jones says. “She has such focus and the ability to not be distracted, which is present in the way she holds herself and pauses before she speaks. She knows how powerful her words can be, so she takes time to formulate those words. I just found her extremely fascinating to watch.”

Reviews have been fairly positive for “Basis” (73 percent on aggregate site Rotten Tomatoes), and IndieWire praised Jones’ “formidable performance,” despite being “saddled with a loose New Yawk accent that comes and goes.” But the only critic she truly cares about is Ginsburg, who has seen the film three times and given her stamp of approval.

“It’s wonderful to see her face watching it and the joy that she has,” Jones says. “I wrote to her and she wrote me back, saying, ‘Your performance is beyond wonderful.’ ... That’s the person I care about liking it.”

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