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Russell’s Kate Wyler also sweats — a lot. Which, like the messy hair, is something you never saw from Elizabeth Jennings, the Soviet spy Russell played with impeccable, delicious cool for six seasons on “The Americans.� Indeed, fans of that FX show will surely gasp at the sight of Russell’s Kate raising her arm so her husband can take a whiff and advise if she needs a shower. Just SO not Elizabeth.

Russell laughs as she confirms that indeed, sweat was foreign to Elizabeth, whose blood ran cold while Kate's runs decidedly hot.

“I always used to think of (Elizabeth) as like a panther,� she said in an interview ahead of the first season of “The Diplomat,� created by Debora Cahn, which drops Thursday. “Very little movement. And I always wore this really cool eyeliner, and my hair was perfect – all very smooth and panthery. This character, Kate, is not that! I’m constantly sweating, the hair is a mess, and it’s probably a lot more like most of us in life."

It’s been five years since we saw Elizabeth in that searing “Americans� finale, standing next to husband Philip (real-life partner Matthew Rhys) and gazing out at Moscow, their covers blown, contemplating a future (spoiler alert!) without their kids. “We’ll get used to it� was her last line, delivered in Russian.

But for some avid fans, it wasn’t so easy to “get used to it,� and they still wonder obsessively what Elizabeth and Philip might be doing these days. The actors were asked that question once again at a 10th-anniversary reunion panel last week at the Paley Center for Media. Russell had a pantherlike response, saying the ending was so perfectly written (by Joel Fields and Joe Weisberg) that she simply preferred to leave it there.

It was also great writing, Russell says, that has brought her back to TV. Busy with three kids, she was definitely not looking for a new show. But then “The Diplomat� came calling. Series creator Cahn is a veteran of both “The West Wing� and “Homeland,� and “The Diplomat� can credibly be seen as a mashup of the two — with some spicy “Veep� humor thrown in — just for starters.

“For me, it’s always about the writing,� Russell says, and “this is so smart and acerbic and full of all this political jargon, but it’s funny, too. (Cahn) has this real take on the minutiae of life and relationships."

And when Russell says the new show is “just lighter,� she doesn’t mean simply that she isn't killing people and stuffing them in suitcases. “I mean, this character is nervous and sweaty and awkward and messy, and it’s fun to get to do that, you know?"

Like many, Cahn was a fan of “The Americans,� and says Russell was the dream choice for Kate — “the moon shot� — an actor with the rare ability to portray power and gravitas, but then turn on a dime to display expert physical comedy.

“From the hair to everything else — falling down and dropping things — and just having an air about her of being on the verge of falling apart all the time,� Cahn says, "that takes a tremendous amount of skill and sense of comedy. And that's what the role needed.�

Not that Kate isn’t competent. A career diplomat, she's about to become ambassador in Kabul when we meet her, a role that would tap her wealth of experience in the region. But then a British aircraft carrier is bombed — by whom, we don't know — and there's no envoy in London. The U.S. president himself (Michael McKean, part of a superbly cast ensemble) asks Kate to take the job, traditionally a political appointment with little substantive responsibility.

Suddenly Kate is living in a palatial English home, and aides are bringing racks of cocktail dresses. Kate does not like dresses. She likes pantsuits, and only black ones, so that when you use your water bottle before a briefing in the Oval to clean the yogurt stain from breakfast, it doesn’t show.

“She is frazzled — a lot,� Russell says of Kate. “But she’s the behind-the-scenes person who will get things done. She’s messy, in a great way.�

Then there’s the marriage. Just as “The Americans� centered on a marriage, “The Diplomat� revolves around Kate’s complex relationship with husband Hal (Rufus Sewell.) An experienced former ambassador himself, Hal isn't used to being “the spouse.�

It’s the layered dynamic of this volatile union (just wait until you see them fighting in the garden) that drives the show, despite its broad global themes. “That’s what you care about,� Russell says. “You want to know how people feel and what’s stressing them out and how they’re living life.� Adds Sewell: “What is the whole globe except billions and billions of little couples, of people? When we both read it, it was that human dynamic, and the humor … that really cracked it open.�

For cast and crew, the experience was also a deep dive into world diplomacy, a subject Cahn first encountered during her “Homeland� research. “Nobody knows these stories because you don’t hear about it,� Cahn says. “If (diplomacy) is done right, nobody ever knows it happened.�

David Gyasi, who plays the British foreign secretary, thought he knew something about diplomacy when he started, but this script was so dense and detailed, he says, that “there were moments where I had to go, ‘Why is this important?’" And then the creative team would launch into a history lesson. “It just opened us all up to another level of diplomacy that was fascinating,� he says.

“What I didn't realize,� notes Ato Essandoh, who plays Kate's top aide, “is how human the interactions are, from the microscopic level of two humans trying to get together and understand each other...to two countries trying to relate to each other.� Adds Ali Ahn, who plays the CIA station chief: “It's all about, do I trust you? Do I like you? Those are the basic building blocks.�

Russell, for research, read “The Ambassadors� by Paul Richter, sharing it with co-star Sewell, who listened to the audiobook on weekend drives. They also watched “The Human Factor,� a documentary about the diplomats involved in Mideast peace negotiations.

“Those guys who were orchestrating those meetings before (Bill) Clinton comes in or before (Yitzhak) Rabin comes — they’re unsung and they’re sort of mysterious," Russell says. "We don’t know about this whole world, and it’s really interesting.�

And so, Russell is relishing her shift to the “good� side.

“By the way, I loved ‘The Americans’, too — it was so fun to play this character who was so much more cool than I was, and wore silk shirts and jewelry," she says. "But this is lighter and snappier, and I'm really enjoying it.�