Christine Baranski’s career is so vast and varied — from starring roles in “The Good Wife” and its spinoff “The Good Fight,” to spending decades in New York City theater, to stealing scenes as the “Mamma Mia” franchise’s resident siren — that it’s genuinely shocking when she points out that she’s never been in an onscreen period piece “with wigs, corsets, language, the whole thing.”

HBO’s “The Gilded Age,” a glamorous new post-Civil War era drama from “Downton Abbey” creator Julian Fellowes, embodies that “whole thing” and then some. So when Fellowes reached out to offer Baranski a part that occupies what many “Downton” fans will recognize as the Dowager Countess role (i.e. the Grand Dame with the most cutting glares and one-liners), she didn’t hesitate. “I mean, who wouldn’t want to play an elitist snob written by Julian Fellowes?” she says. “He does it awfully well.”

“The Gilded Age” sees Fellowes pivoting from early 20th century England to post-Civil War New York City, where old money power players (including Baranski’s formidable Agnes and her more diminutive sister Ada, played by Cynthia Nixon) struggle to keep power from new money interlopers (such as their determined neighbors Bertha and George, played by Carrie Coon and Morgan Spector). After spending years in development, surviving a network change from NBC to HBO, and with filming heavily delayed by COVID, “The Gilded Age” finally premiered on Jan. 24. Baranski can’t wait for audiences to get lost in its world of Edith Wharton culture clashes as much as she did.

When were you first approached about “The Gilded Age”? How did you get involved?

I was at my Connecticut home taking out the recycling bags in July 2019 when I got a call from my manager, who said there was great interest in my doing this show. I had known years ago about Julian [Fellowes] wanting to do, not an American “Downton Abbey,” but something similar in terms of the history. I remember thinking I would do anything, anything, to be in a period piece written by Julian Fellowes.

At one of the awards shows, I was sitting across the aisle from the “Downton Abbey” cast. I was up for a supporting role for “The Good Wife,” and in my category was the great Dame Maggie Smith, so I didn’t have to worry that I would have to get up and speak because I knew she would (and she did). But afterwards there was a party, and I remember going up to Julian and having a conversation. I’m not sure I was cheeky enough to say, “I’d really like to be in something if you wrote it,” but I did express my admiration for “Downton” and spoke something of my husband’s family background being the Drexel family, and what a fascinating period of history it was. So I just remember having a lovely chat with him and I lived in hope that if ever there was something, anything like “Downton Abbey,” that I could be part of it. I’d never done anything historical on film. And so here we are! It’s a dream come true for me.

Agnes does feel like this show’s Dowager Countess.

In a way, yes. She’s certainly got that superior attitude.

And some of the most delicious lines. Were there any you read that made you go, “I can’t wait to say this”?

I think when Marian (Louisa Jacobson) says to me that servants deserve some time off as well, and Agnes turns to her and says, “hwhy?”

I love that “why” has an “h” at the front for her.

Oh yes, that’s how you say it: “hhwhuuyy?” So yes, she’s got some marvelous one-liners and just drips with condescension. But I came to love that character. She was so be horrified by this rampant spending and display of money, this thinking that one could buy your way into society. She sees that kind of degradation in the world happening, and look where we are now, and how far rampant capitalism has gotten us in our world. So I think Aunt Agnes was on to something in her apprehensions.

It’s actually really surprising you’d never done an onscreen period piece before this! I was trying to figure it out before this interview and couldn’t find one, but was sure I must be missing something.

Me, too. I mean, You can’t count something like “The Grinch,” which is fantasy. I mean one of those historical dramas with wigs, corsets, language, the whole thing. I’m sure there wasn’t an actor who didn’t envy the “Downton Abbey” cast and the fact that they got to inhabit that world with the clothes, and the dining table with the crystal and the silver and all that. It was like entering a dream world, and audiences just thought it was the ultimate escape. So I’m hoping American audiences will just delight in revisiting this time in American history following the Civil War, and how transformative society was at that time.

Watching the show, it’s hard not to be taken by the sheer scale of this production. What was it like to shoot in that version of New York versus the other ones that you’ve inhabited throughout your career?

It’s unlike anything I’ve ever done. I mean, this is 1882! But at that time, the Upper East Side was nothing what it looked like now. In fact, we couldn’t shoot in Manhattan because it’s much too modern. We had to do exteriors in Troy, New York to replicate the actual cobblestone streets and such. Then, the Upper East Side was pastoral, except for brownstones like the string Agnes owns — and now, to her horror, this mansion is going up across the street and she’s just horrified. It’s as though a Trump casino is going up outside of her home.

So it was really an imaginative leap to enter that world. I’m happy to say it required a lot of research, a lot of reading. We took classes in etiquette and were given a huge research bible. We had speech sessions for every single scene and worked with a coach to take away that contemporary sound to our voices. Given everything that’s happening in our country and in our world now, it was kind of wonderful to escape to another era.

Especially because your other show (“The Good Fight”) is so purposefully timely — there’s no escaping the world on that one.

You know, we’re all suffering angst and conflict and all of the slings and arrows that this present time in history is serving us. It’s a tough time to be fully aware of what’s going on. Which is not that to say that the world that we’re creating in 1882 [isn’t relevant]. As I said, it was a time of great change, but there was this society that just had a very strict framework for behavior, and a set of expectations for men and women. The changes that came were the beginning of capitalism and that era of great wealth and ferocious change. But yes, I loved getting away from the present tense.

You’re also getting to work with Cynthia Nixon, with whom you’ve worked before. When did you find out she’d be playing your sister?

Oh, Cynthia texted me and said, “is it true that they’ve offered you ‘The Gilded Age’? I might play your sister,” and I said, “yes, oh my God, let’s do this!” We adore each other. 37 years ago, I was pregnant with my firstborn daughter, and Cynthia played my daughter in a Tom Stoppard play called “The Real Thing.” She was a student at Barnard, and I played her mother. Now I’m playing her sister. [laughs] Her much older sister, I might add.

When did “The Gilded Age” shoot?

It was October 2020. We were ready to go in the spring. I was literally having a meeting with the director and Cynthia to discuss character just a few days from starting to shoot. We ran into Julian having a bite to eat in the in the offices, and it was like, “What’s this whole thing about COVID? So ridiculous, we can’t shut down!” And sure enough, a day or two later, everything shut down. It was a very long wait.

I will say that HBO and the powers that be really protected the cast and crew. It’s really a tribute to both my show, “The Good Fight” and “The Gilded Age” that somehow I managed to do 20 episodes of two shows during that period and, knock wood, never got sick. I’m still not trying to be getting sick from this stupid Omicron. But yeah, we managed to do our work. Of course, theater actors did not get to do that, which is why both of my shows happily employed all of these wonderful actors.

There are so many Broadway actors in just about every scene of “The Gilded Age,” I couldn’t believe it. Kate Finneran, Kelli O’Hara, Michael Cerveris, Donna Murphy, Audra McDonald…

Isn’t it incredible? There are so many Tony award winners, so many wonderful actors. If there’s one serendipitous upside to this awful COVID is that, because the theater was shut down, these actors got to be in the show, because we were able to film when you couldn’t perform in a theater. It was just one theater actor after another establishing their characters and bringing their marvelous skillset. Theater actors are rather comfortable stuff like this, it’s what we do. It was kind of like a repertory company, really. I just kept bumping into people and saying, “oh we did this workshop together,” “I saw your play,” “I’ve always loved your work.” It’s a revolving door of great theater actors. And then of course, there’s the introduction of all these newer faces who are so talented, like Louisa and Denae [Benton, who plays Agnes’ secretary and aspiring journalist Peggy Scott]. This will be a very strong showing for new faces, and that’s great as well.

“The Gilded Age” airs Mondays at 9 pm on HBO, and is now available to stream on HBO Max.